Closed-loop load control circuit having a wide output range

ABSTRACT

A load control circuit, such as a light-emitting diode (LED) driver, for controlling the amount of power delivered to an electrical load, such as an LED light source, comprises a regulation transistor adapted to be coupled in series with the load, and a feedback circuit coupled in series with the regulation transistor, whereby the load control circuit is able to control the magnitude of a load current conducted through the load from a minimum load current to a maximum load current, which is at least approximately one thousand times larger than the minimum load current. The feedback circuit generates at least one load current feedback signal representative of the magnitude of the load current. The regulation transistor operates in the linear region to control the magnitude of the load current conducted through the load in response to the magnitude of the load current determined from the load current feedback signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from commonly-assigned U.S. ProvisionalPatent Application No. 61/249,477, filed Oct. 7, 2009, entitled LOADCONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE; U.S. ProvisionalPatent Application No. 61/319,530, filed Mar. 31, 2010; entitled LAMPDRIVER CONFIGURATION DEVELOPMENT TOOL; and U.S. Provisional PatentApplication No. 61/332,983, filed May 10, 2010, entitled LOAD CONTROLDEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosuresof which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a load control device for an electricalload, and more particularly, to an light-emitting diode (LED) driverhaving a load current feedback circuit that allows the LED driver tohave a wide output current range.

2. Description of the Related Art

Light-emitting diode (LED) light sources are often used in place of oras replacements for conventional incandescent, fluorescent, or halogenlamps, and the like. LED light sources may comprise a plurality oflight-emitting diodes mounted on a single structure and provided in asuitable housing. LED light sources are typically more efficient andprovide longer operational lives as compared to incandescent,fluorescent, and halogen lamps. In order to illuminate properly, an LEDdriver control device (i.e., an LED driver) must be coupled between analternating-current (AC) source and the LED light source for regulatingthe power supplied to the LED light source. The LED driver may regulateeither the voltage provided to the LED light source to a particularvalue, the current supplied to the LED light source to a specific peakcurrent value, or may regulate both the current and voltage.

The prior art dealing with LED drivers is extensive. See, for example,the listing of U.S. and foreign patent documents and other publicationsin U.S. Pat. No. 7,352,138, issued Apr. 1, 2008, assigned to PhilipsSolid-State Lighting Solutions, Inc., of Burlington, Mass., and U.S.Pat. No. 6,016,038, issued Jan. 18, 2000, assigned to Color Kinetics,Inc., of Boston, Mass. (hereinafter “CK”).

LED drivers are well known. For example, U.S. Pat. No. 6,586,890, issuedJul. 1, 2003, assigned to Koninklijke Philips Electronics N.V., ofEindhoven, the Netherlands (hereinafter “Philips”), discloses a drivercircuit for LEDs that provide power to the LEDs by using pulse-widthmodulation (PWM). Other examples of LED drives are U.S. Pat. No.6,580,309, published Sep. 27, 2001, assigned to Philips, which describesswitching an LED power supply unit on and off using a pulse durationmodulator to control the mean light output of the LEDs. Moreover, theaforementioned U.S. Pat. No. 6,016,038 also describes using PWM signalsto alter the brightness and color of LEDs. Further, U.S. Pat. No.4,845,481, issued Jul. 4, 1989 to Karel Havel, discloses varying theduty cycles of supply currents to differently colored LEDs to vary thelight intensities of the LEDs so as to achieve continuously variablecolor mixing.

U.S. Pat. No. 6,586,890 also discloses a closed-loop current powersupply for LEDs. Closed-loop current power supplies for supplying powerto other types of lamps are also well known. For example, U.S. Pat. No.5,041,763, issued Aug. 20, 1991, assigned to Lutron Electronics Co.,Inc. of Coopersburg, Pa. (hereinafter “Lutron”), describes closed-loopcurrent power supplies for fluorescent lamps that can supply power toany type of lamp.

U.S. Pat. No. 6,577,512, issued Jun. 10, 2003, assigned to Philips,discloses a power supply for LEDs that uses closed-loop current feedbackto control the current supplied to the LEDs and includes means forprotecting the LEDs. Likewise, U.S. Pat. No. 6,150,771, issued Nov. 21,2000, assigned to Precision Solar Controls Inc., of Garland, Tex., andJapanese patent publication 2001093662A, published 6 Apr. 2001, assignedto Nippon Seiki Co., Ltd., describe over-current and over-voltageprotection for drivers for LEDs and other lamps.

LED drivers that may be dimmed by conventional A.C. dimmers are alsoknown. Thus, aforementioned U.S. Pat. No. 7,352,138, and U.S. Pat. No.7,038,399, issued May 2, 2006, assigned to CK, describe LED-based lightsources that are controlled by conventional A.C. phase control dimmers.The aforementioned U.S. Pat. No. 6,016,038 discloses a PWM controlledLED-based light source used as a light bulb that may be placed in anEdison-mount (screw-type) light bulb housing. Control of lamps, such asLED lamps, by phase control signals are also described in U.S. Pat. No.6,111,368, issued Aug. 29, 2000, U.S. Pat. No. 5,399,940, issued Mar.21, 1995, U.S. Pat. No. 5,017,837, issued May 21, 1991, all of which areassigned to Lutron. U.S. Pat. No. 6,111,368, for example, discloses anelectronic dimming fluorescent lamp ballast that is controlled by aconventional A.C. phase control dimmer. U.S. Pat. No. 5,399,940discloses a microprocessor-controlled “smart” dimmer that controls thelight intensities of an array of LEDs in response to a phase controldimming voltage waveform. U.S. Pat. No. 5,017,837 discloses an analogA.C. phase control dimmer having an indicator LED, the intensity ofwhich is controlled in response to a phase control dimming voltagewaveform. The well-known CREDENZA® in-line lamp cord dimmer,manufactured by Lutron since 1977, also includes an indicator LED, thelight intensity of which is controlled in response to a phase controldimming voltage waveform.

Applications for LED illumination systems are also shown in U.S. Pat.No. 7,309,965, issued Dec. 18, 2007, and U.S. Pat. No. 7,242,152, issuedJul. 10, 2007, both assigned to CK. U.S. Pat. No. 7,309,965 disclosessmart lighting devices having processors, and networks comprising suchsmart lighting devices, sensors, and signal emitters. U.S. Pat. No.7,242,152 discloses systems and methods for controlling a plurality ofnetworked lighting devices in response to lighting control signals. Suchsystems are also used in the RADIORA® product, which has been sold since1996 by Lutron.

In addition, there are known techniques for controlling currentdelivered to an LED light source. LED light sources are often referredto as “LED light engines.” These LED light engines typically comprise aplurality of individual LED semiconductor structures, such as, forexample, Gallium-Indium-Nitride (GaInN) LEDs. The individual LEDs mayeach produce light photons by electron-hole combination in the bluevisible spectrum, which is converted to white light by a yellow phospherfilter.

It is known that the light output of an LED is proportional to thecurrent flowing through it. It is also known that LEDs suffer from aphenomena known as “droop” in which the efficiency is reduced as thepower is increased. For LEDs of the GaInN type (used for providingillumination), a typical load current is approximately 350 milliamps(mA) at a forward operating voltage of between three and four volts (V)which corresponds to approximately a one watt (W) power rating. At thispower rating, these LEDs provide approximately 100 lumens per watt. Thisis significantly more efficient than other conventional light sources.For example, incandescent lamps typically provide 10 to 20 lumens perwatt and fluorescent lamps, 60 to 90 lumens per watt. As discussed, LEDlight sources can provide larger ratios of lumens per watt at lowercurrents, thus avoiding the droop phenomena. Further, it is expectedthat, as technology improves, the efficiency of LED light sources willimprove even at higher current levels than presently employed to providehigher light outputs per diode in an LED light engine.

LED light sources typically comprise a plurality of individual LEDs thatmay be arranged in both a series and parallel relationship. In otherwords, a plurality of LEDs may be arranged in a series string and anumber of series strings may be arranged in parallel to achieve thedesired light output. For example, five LEDs in a first series stringeach with a forward bias of approximately 3 volts (V) and each consumingapproximately one watt of power (at 350 mA through the string) consumeabout 5 W. A second string of a series of five LEDs connected inparallel across the first string will result in a power consumption of10 W with each string drawing 350 mA. Thus, an LED driver would need tosupply 700 mA to the two strings of LEDs, and since each string has fiveLEDs, the output voltage provided by the LED driver would be about 15volts. Additional strings of LEDs can be placed in parallel foradditional light output, however, the LED driver must be operable toprovide the necessary current. Alternatively, more LEDs can be placed inseries on each sting, and as a result, the LED driver must also beoperable to provide the necessary voltage (e.g., 18 volts for a seriesof six LEDs).

LED light sources are typically rated to be driven via one of twodifferent control techniques: a current load control technique or avoltage load control technique. An LED light source that is rated forthe current load control technique is also characterized by a ratedcurrent (e.g., 350 milliamps) to which the peak magnitude of the currentthrough the LED light source should be regulated to ensure that the LEDlight source is illuminated to the appropriate intensity and color. Incontrast, an LED light source that is rated for the voltage load controltechnique is characterized by a rated voltage (e.g., 15 volts) to whichthe voltage across the LED light source should be regulated to ensureproper operation of the LED light source. Typically, each string of LEDsin an LED light source rated for the voltage load control techniqueincludes a current balance regulation element to ensure that each of theparallel legs has the same impedance so that the same current is drawnin each parallel string.

In addition, it is known that the light output of an LED light sourcecan be dimmed. Different methods of dimming LEDs include a pulse-widthmodulation (PWM) technique and a constant current reduction (CCR)technique. Pulse-width modulation dimming can be used for LED lightsources that are controlled in either a current or voltage load controlmode. In pulse-width modulation dimming, a pulsed signal with a varyingduty cycle is supplied to the LED light source. If an LED light sourceis being controlled using the current load control technique, the peakcurrent supplied to the LED light source is kept constant during an ontime of the duty cycle of the pulsed signal. However, as the duty cycleof the pulsed signal varies, the average current supplied to the LEDlight source also varies, thereby varying the intensity of the lightoutput of the LED light source. If the LED light source is beingcontrolled using the voltage load control technique, the voltagesupplied to the LED light source is kept constant during the on time ofthe duty cycle of the pulsed signal in order to achieve the desiredtarget voltage level, and the duty cycle of the load voltage is variedin order to adjust the intensity of the light output. Constant currentreduction dimming is typically only used when an LED light source isbeing controlled using the current load control technique. In constantcurrent reduction dimming, current is continuously provided to the LEDlight source, however, the DC magnitude of the current provided to theLED light source is varied to thus adjust the intensity of the lightoutput.

Therefore, there is a need to provide an LED driver that is flexible andconfigurable, such that it can be used with LED light sources that arerated to operate at different voltage and current magnitudes, and usingthe different load control and dimming techniques. In addition, there isa need to provide an LED driver that is more efficient and is relativelysimple with a reduced component count. There is a need for a simplerdriver regulator circuit that is also energy efficient. Furthermore,there is a need for an LED driver that maximizes efficiency of thedriver by reducing losses in the driver itself.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a load controlcircuit for controlling the amount of power delivered to an electricalload comprises a regulation transistor adapted to be coupled in serieswith the load, and a feedback circuit coupled in series with theregulation transistor, whereby the load control circuit is able tocontrol the magnitude of a load current conducted through the load froma minimum load current to a maximum load current, which is at leastapproximately one thousand times larger than the minimum load current.The feedback circuit generates a load current feedback signalrepresentative of the magnitude of the load current. The regulationtransistor operates in the linear region to control the magnitude of theload current conducted through the load in response to the magnitude ofthe load current determined from the load current feedback signal, so asto control the amount of power delivered to the load, such that themaximum load current is at least approximately one thousand times largerthan the minimum load current.

According to another embodiment of the present invention, an LED driverfor controlling an LED light source comprises a power converter circuitoperable to receive a rectified AC voltage and to generate a DC busvoltage, and a LED drive circuit operable to receive the bus voltage andto control the magnitude of a load current conducted through the LEDlight source. The LED drive circuit comprises a feedback circuitoperable to generate a first load current feedback signal representativeof the magnitude of the load current. The LED driver further comprises acontrol circuit operatively coupled to the LED drive circuit forcontrolling the magnitude of the load current through the load inresponse to the first load current feedback signal, such that the loadcontrol circuit is able to control the magnitude of the load currentconducted through the load from a minimum load current to a maximum loadcurrent, and the maximum load current is at least approximately onehundred times larger than the minimum load current

According to another aspect of the present invention, an LED driver forcontrolling an LED light source comprises a power converter circuitoperable to receive a rectified AC voltage and to generate a DC busvoltage, and a LED drive circuit operable to receive the bus voltage andto control both the magnitude of a load current conducted through theLED light source and the magnitude of a load voltage produced across theLED light source. The LED driver further comprises a control circuitcoupled to the LED drive circuit for adjusting the magnitude of the loadcurrent conducted through the LED light source when operating in acurrent load control mode, and adjusting the magnitude of the loadvoltage produced across the LED light source when operating in a voltageload control mode.

In addition, a load control circuit for controlling the amount of powerdelivered to an electrical load is also described herein. The loadcontrol circuit comprises a regulation transistor adapted to be coupledin series with the load to control the magnitude of a load currentconducted through the load, so as to control the amount of powerdelivered to the load, a feedback circuit coupled in series with theregulation transistor and operable to generate first and second loadcurrent feedback signals representative of the magnitude of the loadcurrent, and a control circuit operable to determine the magnitude ofthe load current in response to both the first and second load currentfeedback signals. The first and second load current feedback signals arecharacterized by respective first and second gains with respect to themagnitude of the load current, the first gain different than the secondgain. The control circuit is operatively coupled to the regulationtransistor for controlling the regulation transistor to operate in thelinear region to thus adjust the magnitude of the load current throughthe load in response to the magnitude of the load current determinedfrom the first and second load current feedback signals.

According to another embodiment of the present invention, a load controlcircuit for controlling the amount of power delivered to an electricalload comprises a regulation transistor adapted to be coupled in serieswith the load to control the magnitude of a load current conductedthrough the load, so as to control the amount of power delivered to theload, an adjustable-gain feedback circuit coupled in series with theregulation FET and operable to generate a load current feedback signalrepresentative of the magnitude of the load current, and a controlcircuit operatively coupled to the regulation transistor for controllingthe regulation transistor to thus adjust the magnitude of the loadcurrent through the load. The adjustable-gain feedback circuit comprisesfirst and second resistors coupled in series with the regulation FET,and a gain-adjustment transistor coupled across the second resistor. Thecontrol circuit is further coupled to the adjustable-gain feedbackcircuit for rendering the gain-adjustment transistor conductive andnon-conductive, such that the series combination of the first and secondresistors is coupled in series with the regulation FET when thegain-adjustment transistor is non-conductive, and only the firstresistor is coupled in series with the regulation FET when thegain-adjustment transistor is conductive. The control circuit rendersthe gain-adjustment transistor non-conductive when the magnitude of theload current is less than a threshold current.

The present invention further provides a method of controlling theamount of power delivered to an electrical load. The method comprises(1) controlling the magnitude of a load current conducted through theload, so as to control the amount of power delivered to the load; (2)generating first and second load current feedback signals representativeof the magnitude of the load current, the first and second load currentfeedback signals characterized by respective first and second gainsapplied to the magnitude of the load current, the first gain differentthan the second gain; (3) calculating the magnitude of the load currentin response to both the first and second load current feedback signals;and (4) adjusting the magnitude of the load current in response to thecalculated magnitude of the load current determined from the first andsecond load current feedback signals.

According to another embodiment of the present invention, a method ofcontrolling the amount of power delivered to an electrical loadcomprises: (1) controlling the magnitude of a load current conductedthrough the load, so as to control the amount of power delivered to theload; (2) conducting the load current through first and secondseries-connected resistors; (3) generating a load current feedbacksignal across the series-connected resistors, the load current feedbacksignal representative of the magnitude of the load current; (4)calculating the magnitude of the load current in response to both theload current feedback signal; (5) adjusting the magnitude of the loadcurrent in response to the magnitude of the load current determined fromthe first and second load current feedback signals; (6) controlling again-adjustment transistor coupled across the second resistor to beconductive, such that the load current feedback signal is generated fromonly the first resistor; and (7) controlling the gain-adjustmenttransistor coupled across the second resistor to be non-conductive whenthe magnitude of the load current is less than a threshold current, suchthat the load current feedback signal is generated across the seriescombination of the first and second resistors.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a system including alight-emitting diode (LED) driver for controlling the intensity of anLED light source according to a first embodiment of the presentinvention;

FIG. 2 is a simplified block diagram of the LED driver of FIG. 1;

FIG. 3 is a simplified schematic diagram of a flyback converter and anLED drive circuit of the LED driver of FIG. 1;

FIGS. 4A and 4B are simplified flowcharts of a startup procedureexecuted by a control circuit of the LED driver of FIG. 1;

FIG. 5 is a simplified flowchart of a target intensity procedureexecuted by the control circuit of the LED driver of FIG. 1;

FIG. 6 is a simplified flowchart of a current load control modeprocedure executed by the control circuit of the LED driver of FIG. 1 ina current load control mode;

FIG. 7 is a simplified flowchart of a voltage load control modeprocedure executed by the control circuit of the LED driver of FIG. 1 ina voltage load control mode;

FIG. 8 is a simplified schematic diagram of an LED drive circuit of anLED driver according to a second embodiment of the present invention;

FIG. 9 is a simplified flowchart of a transition mode procedure executedperiodically by a control circuit of the LED driver of FIG. 8 accordingto the second embodiment of the present invention;

FIG. 10 is a simplified block diagram of an LED driver according to athird embodiment of the present invention;

FIG. 11 is a simplified circuit diagram of a flyback converter of theLED driver of FIG. 10 according to the third embodiment of the presentinvention;

FIG. 12 is a simplified schematic diagram of an LED drive circuit of theLED driver of FIG. 10 according to the third embodiment of the presentinvention;

FIG. 13 is a simplified flowchart of a load current feedback procedureexecuted by a control circuit of the LED driver of FIG. 10 when the LEDdriver is operating in the current load control mode;

FIG. 14 is a simplified schematic diagram of an LED drive circuit of aLED driver according to a fourth embodiment of the present invention;

FIG. 15A is a plot of a duty cycle of a load current with respect to thetarget intensity of the LED driver of FIG. 14 according to the fourthembodiment of the present invention;

FIG. 15B is a plot of a peak magnitude of the load current with respectto the target intensity of the LED driver of FIG. 14 according to thefourth embodiment of the present invention;

FIG. 16 is a simplified flowchart of a target intensity procedureexecuted by a control circuit of the LED driver of FIG. 14 according tothe fourth embodiment of the present invention;

FIG. 17 is a simplified flowchart of a transition mode procedureexecuted periodically by the control circuit of the LED driver of FIG.14 according to the fourth embodiment of the present invention;

FIG. 18 is a simplified block diagram of an LED driver developmentsystem;

FIG. 19 is a simplified block diagram of a portion of the system of FIG.18;

FIG. 20 is an example of a display screen presented by software thatoperates on a computer in the system of FIG. 18;

FIG. 21 is a general flowchart of the operation of the system of FIG.18;

FIG. 22 is a simplified software flowchart of a configuration processexecuted by the computer of the system of FIG. 18; and

FIG. 23 is a simplified software flowchart of the configuration processexecuted by the LED driver while being configured in the system of FIG.18.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 1 is a simplified block diagram of a system including alight-emitting diode (LED) driver 100 for controlling the intensity ofan LED light source 102 (e.g., an LED light engine) according to a firstembodiment of the present invention. The LED light source 102 is shownas a plurality of LEDs connected in series but may comprise a single LEDor a plurality of LEDs connected in parallel or a suitable combinationthereof, depending on the particular lighting system. In addition, theLED light source 102 may alternatively comprise one or more organiclight-emitting diodes (OLEDs). The LED driver 100 is coupled to analternating-current (AC) power source 104 via a dimmer switch 106. Thedimmer switch 106 generates a phase-control signal V_(PC) (e.g., adimmed-hot voltage), which is provided to the LED driver 100. The dimmerswitch 106 comprises a bidirectional semiconductor switch (not shown),such as, for example, a triac or two anti-series-connected field-effecttransistors (FETs), coupled in series between the AC power source 104and the LED driver 100. The dimmer switch 106 controls the bidirectionalsemiconductor switch to be conductive for a conduction period T_(CON)each half-cycle of the AC power source 104 to generate the phase-controlsignal V_(PC).

The LED driver 100 is operable to turn the LED light source 102 on andoff in response to the conductive period T_(CON) of the phase-controlsignal V_(PC) received from the dimmer switch 106. In addition, the LEDdriver 100 is operable to adjust (i.e., dim) the intensity of the LEDlight source 102 to a target intensity L_(TRGT), which may range acrossa dimming range of the LED light source, i.e., between a low-endintensity L_(LE) (e.g., approximately 1%) and a high-end intensityL_(HE) (e.g., approximately 100%) in response to the phase-controlsignal V_(PC). The LED driver 100 is able to control both the magnitudeof a load current I_(LOAD) through the LED light source 102 and themagnitude of a load voltage V_(LOAD) across the LED light source.Accordingly, the LED driver 100 controls at least one of the loadvoltage V_(LOAD) across the LED light source 102 and the load currentI_(LOAD) through the LED light source to control the amount of powerdelivered to the LED light source depending upon a mode of operation ofthe LED driver (as will be described in greater detail below).

The LED driver 100 is adapted to work with a plurality of different LEDlight sources, which may be rated to operate using different loadcontrol techniques, different dimming techniques, and differentmagnitudes of load current and voltage. The LED driver 100 is operableto control the magnitude of the load current I_(LOAD) through the LEDlight source 102 or the load voltage V_(LOAD) across the LED lightsource using two different modes of operation: a current load controlmode (i.e., for using the current load control technique) and a voltageload control mode (i.e., for using the voltage load control technique).The LED driver 100 may also be configured to adjust the magnitude towhich the LED driver will control the load current I_(LOAD) through theLED light source 102 in the current load control mode, or the magnitudeto which the LED driver will control the load voltage V_(LOAD) acrossthe LED light source in the voltage load control mode. When operating inthe current load control mode, the LED driver 100 is operable to controlthe intensity of the LED light source 102 using two different dimmingmodes: a PWM dimming mode (i.e., for using the PWM dimming technique)and a CCR dimming mode (i.e., for using the CCR dimming technique). Whenoperating in the voltage load control mode, the LED driver 100 is onlyoperable to adjust the amount of power delivered to the LED light source102 using the PWM dimming technique.

FIG. 2 is a simplified block diagram of the LED driver 100 according tothe first embodiment of the present invention. The LED driver 100comprises a radio-frequency (RFI) filter and rectifier circuit 110,which receives the phase-control signal V_(PC) from the dimmer switch106. The RFI filter and rectifier circuit 110 operates to minimize thenoise provided on the AC power source 104 and to generate a rectifiedvoltage V_(RECT). The LED driver 100 further comprises a powerconverter, e.g., a buck-boost flyback converter 120, which receives therectified voltage V_(RECT) and generates a variable direct-current (DC)bus voltage V_(BUS) across a bus capacitor C_(BUS). The flybackconverter 120 may alternatively comprise any suitable power convertercircuit for generating an appropriate bus voltage. The bus voltageV_(BUS) may be characterized by some voltage ripple as the bus capacitorC_(BUS) periodically charges and discharges. The flyback converter 120may also provide electrical isolation between the AC power source 104and the LED light source 102, and operate as a power factor correction(PFC) circuit to adjust the power factor of the LED driver 100 towards apower factor of one. Alternatively, the flyback converter 120 couldcomprise a boost converter, a buck converter, a single-endedprimary-inductor converter (SEPIC), a Ćuk converter, or other suitablepower converter circuit.

The LED driver 100 also comprises an LED drive circuit 130, whichreceives the bus voltage V_(BUS) and controls the amount of powerdelivered to the LED light source 102 so as to control the intensity ofthe LED light source. The LED drive circuit 130 may comprise acontrollable-impedance circuit, such as a linear regulator, as will bedescribed in greater detail below. Alternatively, the LED drive circuit130 could comprise a switching regulator, such as a buck converter.

The LED driver 100 further comprises a control circuit 140 forcontrolling the operation of the flyback converter 120 and the LED drivecircuit 130. The control circuit 140 may comprise, for example, amicrocontroller or any other suitable processing device, such as, forexample, a programmable logic device (PLD), a microprocessor, or anapplication specific integrated circuit (ASIC). The LED driver 100further comprises a power supply 150, which receives the rectifiedvoltage V_(RECT) and generates a plurality of direct-current (DC) supplyvoltages for powering the circuitry of the LED driver. Specifically, thepower supply 150 generates a first non-isolated supply voltage V_(CC1)(e.g., approximately 14 volts) for powering the control circuitry of theflyback converter 120, a second isolated supply voltage V_(CC2) (e.g.,approximately 9 volts) for powering the control circuitry of the LEDdrive circuit 130, and a third non-isolated supply voltage V_(CC3)(e.g., approximately 5 volts) for powering the control circuit 140.

The control circuit 140 is coupled to a phase-control input circuit 160,which generates a target intensity control signal V_(TRGT). The targetintensity control signal V_(TRGT) comprises, for example, a square-wavesignal having a duty cycle DC_(TRGT), which is dependent upon theconduction period T_(CON) of the phase-control signal V_(PC) receivedfrom the dimmer switch 106, and thus is representative of the targetintensity L_(TRGT) of the LED light source 102. Alternatively, thetarget intensity control signal V_(TRGT) could comprise a DC voltagehaving a magnitude dependent upon the conduction period T_(CON) of thephase-control signal V_(PC), and thus representative of the targetintensity L_(TRGT) of the LED light source 102.

The control circuit 140 is also coupled to a memory 170 for storing theoperational characteristics of the LED driver 100 (e.g., the loadcontrol mode, the dimming mode, and the magnitude of the rated loadvoltage or current). Finally, the LED driver 100 may also comprise acommunication circuit 180, which may be coupled to, for example, a wiredcommunication link or a wireless communication link, such as aradio-frequency (RF) communication link or an infrared (IR)communication link. The control circuit 140 may be operable to updatethe target intensity L_(TRGT) of the LED light source 102 or theoperational characteristics stored in the memory 170 in response todigital messages received via the communication circuit 180. Forexample, the LED driver 100 could alternatively be operable to receive afull conduction AC waveform directly from the AC power source 104 (i.e.,not the phase-control signal V_(PC) from the dimmer switch 106) andcould simply determine the target intensity L_(TRGT) for the LED lightsource 102 from the digital messages received via the communicationcircuit 180.

As previously mentioned, the control circuit 140 manages the operationof the flyback converter 120 and the LED drive circuit 130 to controlthe intensity of the LED light source 102. The control circuit 140receives a bus voltage feedback signal V_(BUS-FB), which isrepresentative of the magnitude of the bus voltage V_(BUS), from theflyback converter 120. The control circuit 140 provides a bus voltagecontrol signal V_(BUS-CNTL) to the flyback converter 120 for controllingthe magnitude of the bus voltage V_(BUS) (e.g., from approximately 8volts to 60 volts). When operating in the current load control mode, theLED drive circuit 130 controls a peak magnitude I_(PK) of the loadcurrent I_(LOAD) conducted through the LED light source 102 between aminimum load current I_(LOAD-MIN) and a maximum load currentI_(LOAD-MAX) in response to a peak current control signal V_(IPK)provided by the control circuit 140. The control circuit 140 receives aload current feedback signal V_(ILOAD), which is representative of themagnitude of the load current I_(LOAD) flowing through the LED lightsource 102. The control circuit 140 also receives a LED voltage feedbacksignal V_(LED-NEG), which is representative of the magnitude of thevoltage at the negative terminal of the LED light source 102. Thecontrol circuit 140 is operable to calculate the magnitude of a loadvoltage V_(LOAD) developed across the LED light source 102 in responseto the bus voltage feedback signal V_(BUS-FB) and the LED voltagefeedback signal V_(LED-NEG) as will be described in greater detailbelow.

The control circuit 140 is operable to control the LED drive circuit130, so as to control the amount of power delivered to the LED lightsource 102 using the two different modes of operation (i.e., the currentload control mode and the voltage load control mode). During the currentload control mode, the LED drive circuit 130 regulates the peakmagnitude I_(PK) of the load current I_(LOAD) through the LED lightsource 102 to a target load current I_(TRGT) in response to the loadcurrent feedback signal V_(ILOAD) (i.e., using closed loop control). Thetarget load current I_(TRGT) may be stored in the memory 170 and may beprogrammed to be any specific magnitude depending upon the LED lightsource 102 (as will be described in greater detail below with referenceto FIGS. 18-23).

To control the intensity of the LED light source 102 during the currentload control mode, the control circuit 140 is operable to control theLED drive circuit 130 to adjust the amount of power delivered to the LEDlight source 102 using both of the dimming techniques (i.e., the PWMdimming technique and the CCR dimming technique). Using the PWM dimmingtechnique, the control circuit 140 controls the peak magnitude I_(PK) ofthe load current I_(LOAD) through the LED light source 102 to the targetload current I_(TRGT) and then pulse-width modulates the load currentI_(LOAD) to dim the LED light source 102 to achieve the target loadcurrent I_(TRGT). Specifically, the LED drive circuit 130 controls aduty cycle DC_(ILOAD) of the load current I_(LOAD) in response to a dutycycle DC_(DIM) of a dimming control signal V_(DIM) provided by thecontrol circuit 140. Accordingly, the intensity of the LED light source102 is dependent upon the duty cycle DC_(ILOAD) of the pulse-widthmodulated load current I_(LOAD). Using the CCR technique, the controlcircuit 140 does not pulse-width modulate the load current I_(LOAD), butinstead adjusts the magnitude of the target load current I_(TRGT) so asto adjust the DC magnitude of the load current I_(LOAD) through the LEDlight source 102.

During the voltage load control mode, the LED drive circuit 130regulates the DC voltage of the load voltage V_(LOAD) across the LEDlight source 102 to a target load voltage V_(TRGT). The target loadvoltage V_(TRGT) may be stored in the memory 170 and may be programmedto be any specific magnitude depending upon the LED light source 102 (aswill be described in greater detail below with reference to FIGS.18-23). The control circuit 140 is operable to dim the LED light source102 using only the PWM dimming technique during the voltage load controlmode. Specifically, the control circuit 140 adjusts a duty cycleDC_(VLOAD) of the load voltage V_(LOAD) to dim the LED light source 102.

FIG. 3 is a simplified schematic diagram of the flyback converter 120and the LED drive circuit 130. The flyback converter 120 comprises aflyback transformer 210 having a primary winding coupled in series witha flyback switching transistor, e.g., a field-effect transistor (FET)Q212 or other suitable semiconductor switch. The secondary winding ofthe flyback transformer 210 is coupled to the bus capacitor C_(BUS) viaa diode D214. The bus voltage feedback signal V_(BUS-FB) is generated bya voltage divider comprising two resistors R216, R218 coupled across thebus capacitor C_(BUS). A flyback controller 222 receives the bus voltagecontrol signal V_(BUS-CNTL) from the control circuit 140 via a filtercircuit 224 and an optocoupler circuit 226, which provides electricalisolation between the flyback converter 120 and the control circuit 140.The flyback controller 222 may comprise, for example, part numberTDA4863, manufactured by Infineon Technologies. The filter circuit 224may comprise, for example, a two-stage resistor-capacitor (RC) filter,for generating a filtered bus voltage control signal V_(BUS-CNTL), whichhas a DC magnitude dependent upon a duty cycle DC_(BUS) of the busvoltage control signal V_(BUS-CNTL). The flyback controller 222 alsoreceives a control signal representative of the current through the FETQ212 from a feedback resistor R228, which is coupled in series with theFET.

The flyback controller 222 controls the FET Q212 to selectively conductcurrent through the flyback transformer 210 to thus generate the busvoltage V_(BUS). The flyback controller 222 is operable to render theFET Q212 conductive and non-conductive at a high frequency (e.g.,approximately 150 kHz or less) to thus control the magnitude of the busvoltage V_(BUS) in response to the DC magnitude of the filtered busvoltage control signal V_(BUS-F) and the magnitude of the currentthrough the FET Q212. Specifically, the control circuit 140 increasesthe duty cycle DC_(BUS) of the bus voltage control signal V_(BUS-CNTL),such that the DC magnitude of the filter bus voltage control signalV_(BUS-F) increases in order to decrease the magnitude of the busvoltage V_(BUS). The control circuit 140 decreases the duty cycleDC_(BUS) of the bus voltage control signal V_(BUS-CNTL) to increase themagnitude of the bus voltage V_(BUS).

As previously mentioned, the LED drive circuit 130 comprises a linearregulator (i.e., a controllable-impedance circuit) including a powersemiconductor switch, e.g., a regulation field-effect transistor (FET)Q232, coupled in series with the LED light source 102 for conducting theload current I_(LOAD). The regulation FET Q232 could alternativelycomprise a bipolar junction transistor (BJT), an insulated-gate bipolartransistor (IGBT), or any suitable transistor. The peak current controlsignal V_(IPK) is coupled to the gate of the regulation FET Q232 througha filter circuit 234, an amplifier circuit 236, and a gate resistorR238. The control circuit 140 is operable to control a duty cycleDC_(IPK) of the peak current control signal V_(IPK) to control themagnitude of the load current I_(LOAD) conducted through the LED lightsource 102 to the target load current I_(TRGT). The filter circuit 234(e.g., a two-stage RC filter) generates a filtered peak current controlsignal V_(IPK-F), which has a DC magnitude dependent upon the duty cycleDC_(IPK) of the peak current control signal V_(IPK), and is thusrepresentative of the magnitude of the target load current I_(TRGT). Theamplifier circuit 236 generates an amplified peak current control signalV_(IPK-A), which is provided to the gate of the regulation transistorQ232 through the resistor R238, such that a gate voltage V_(IPK-G) atthe gate of the regulation transistor Q232 has a magnitude dependentupon the target load current I_(TRGT). The amplifier circuit 236 maycomprise a standard non-inverting operational amplifier circuit having,for example, a gain a of approximately three.

A feedback circuit 242 comprising a feedback resistor R244 is coupled inseries with the regulation FET Q232, such that the voltage generatedacross the feedback resistor is representative of the magnitude of theload current I_(LOAD). For example, the feedback resistor R244 may havea resistance of approximately 0.0375Ω. The feedback circuit 240 furthercomprises a filter circuit 246 (e.g., a two-stage RC filter) coupledbetween the feedback resistor R244 and an amplifier circuit 248 (e.g., anon-inverting operational amplifier circuit having a gain β ofapproximately 20). Alternatively, the amplifier circuit 248 could have avariable gain, which could be controlled by the control circuit 140 andcould range between approximately 1 and 1000. The amplifier circuit 248generates the load current feedback signal V_(ILOAD), which is providedto the control circuit 140 and is representative of an average magnitudeI_(AVE) of the load current I_(LOAD), e.g.,I _(AVE) =V _(ILOAD)/(β·R _(FB)),  (Equation 1)wherein R_(FB) is the resistance of the feedback resistor R244. Whenoperating in the current load control mode, the control circuit 140controls the regulation FET Q232 to operate in the linear region, suchthat the magnitude of the load current I_(LOAD) is dependent upon the DCmagnitude of the filtered peak current control signal V_(IPK-F). Inother words, the regulation FET Q232 provides a controllable-impedancein series with the LED light source 102. When operating in the voltageload control mode, the control circuit 140 is operable to drive theregulation FET Q232 into the saturation region, such that the magnitudeof the load voltage V_(LOAD) is approximately equal to the magnitude ofthe bus voltage V_(BUS) (minus the small voltage drops due to theon-state drain-source resistance R_(DS-ON) of the FET regulation Q232and the resistance of the feedback resistor R244).

The LED drive circuit 130 also comprises a dimming FET Q250, which iscoupled between the gate of the regulation FET Q232 and circuit common.The dimming control signal V_(DIM) from the control circuit 140 isprovided to the gate of the dimming FET Q250. When the dimming FET Q250is rendered conductive, the regulation FET Q232 is renderednon-conductive, and when the dimming FET Q250 is renderednon-conductive, the regulation FET Q232 is rendered conductive. Whileusing the PWM dimming technique during the current mode of operation,the control circuit 140 adjusts the duty cycle DC_(DIM) of the dimmingcontrol signal V_(DIM) to thus control the intensity of the LED lightsource 102. As the duty cycle DC_(DIM) of the dimming control signalV_(DIM) increases, the duty cycle DC_(ITRGT), DC_(VTRGT) of thecorresponding load current I_(LOAD) or load voltage V_(LOAD) decreases,and vice versa. When using the PWM dimming technique in both the currentand voltage load control modes, the control circuit 140 is operable tocalculate the peak magnitude I_(PK) of the load current I_(LOAD) fromthe load current feedback signal V_(ILOAD) (which is representative ofthe average magnitude I_(AVE) of the load current I_(LOAD)) and the dutycycle DC_(DIM) of the dimming control signal V_(DIM), i.e.,I _(PK) =I _(AVE)/(1−DC _(DIM)).  (Equation 2)

When using the CCR dimming technique during the current mode ofoperation, the control circuit 140 maintains the duty cycle DC_(DIM) ofthe dimming control signal V_(DIM) at a high-end dimming duty cycleDC_(HE) (e.g., approximately 0%, such that the FET Q232 is alwaysconductive) and adjusts the target load current I_(TRGT) (via the dutycycle DC_(IPK) of the peak current control signal V_(IPK)) to controlthe intensity of the LED light source 102.

The LED voltage feedback signal V_(LED-NEG) is generated by a voltagedivider comprising two resistors R260, R262 coupled to the negativeterminal of the LED light source 102, such that the magnitude of the LEDvoltage feedback signal V_(LED-NEG) is representative of a regulatorvoltage V_(REG) generated across the series combination of theregulation FET Q232 and the feedback resistor R242. The control circuit140 is operable to calculate the magnitude of a load voltage V_(LOAD)developed across the LED light source 102 in response to the bus voltagefeedback signal V_(BUS-FB) and the LED voltage feedback signalV_(LED-NEG).

When operating in the current load control mode, the control circuit 140is operable to adjust the magnitude of the bus voltage V_(BUS) tocontrol the magnitude of the regulator voltage V_(REG) to a targetregulator voltage V_(REG-TRGT) (i.e., a minimum or “drop-out” voltage,such as, for example, approximately two volts). By controlling theregulator voltage V_(REG) to the target regulator voltage V_(REG-TRGT),the control circuit 140 is able to minimize the magnitude of theregulator voltage (and thus the power dissipated in the regulation FETQ232) as well as ensuring that the regulator voltage does not drop toolow and the load voltage V_(LOAD) does not have any voltage ripple.Accordingly, the control circuit 140 is operable to optimize theefficiency and reduce the total power dissipation of the LED driver 100by controlling the magnitude of the bus voltage V_(BUS), such that thepower dissipation is optimally balanced between the flyback converter120 and the LED drive circuit 130. In other words, the control circuit140 is operable to adjust the magnitude of the bus voltage V_(BUS) inorder to reduce the total power dissipation in the flyback converter 120and the LED drive circuit 130. In addition, since the load voltageV_(LOAD) does not have any voltage ripple, the peak magnitude I_(PK) ofthe load current I_(LOAD) and thus the intensity of the LED light source102 is maintained constant.

FIGS. 4A and 4B are simplified flowcharts of a startup procedure 300executed by the control circuit 140 of the LED driver 100 when thecontrol circuit first starts up at step 310 (e.g., when the LED driver100 is first powered up). If the LED driver 100 is operating in thecurrent load control mode (as stored in the memory 170) at step 312, thecontrol circuit 140 determines if the target load current I_(TRGT) andthe dimming method are known (i.e., are stored in the memory 170) atsteps 314, 316. If the target load current I_(TRGT) and the dimmingmethod are known at steps 314, 316, and the dimming method is the PWMdimming technique at step 318, the control circuit 140 sets the dutycycle DC_(DIM) of the dimming control signal V_(DIM) equal to a low-enddimming duty cycle DC_(LE) at step 320. For example, the low-end dutycycle DC_(LE) may be approximately 99%, such that the dimming FET Q250is rendered conductive 99% of the time, thus causing the regulation FETQ232 to be rendered conductive approximately 1% of the time (i.e., tocontrol the intensity of the LED light source 102 to the low-endintensity L_(LE)). If the dimming method is the CCR dimming technique atstep 318, the control circuit 140 sets the duty cycle DC_(DIM) of thedimming control signal V_(DIM) equal to the high-end dimming duty cycleDC_(HE) (i.e., approximately 0%) at step 322. The control circuit 140then sets the duty cycle DC_(IPK) of the peak current control signalV_(IPK) to a minimum peak current duty cycle DC_(MIN) at step 324.

Next, the control circuit 140 executes a current load control procedure500 (which will be described in greater detail below with reference toFIG. 6) in order to regulate the peak magnitude I_(PK) of the loadcurrent I_(LOAD) flowing through the feedback resistor R242 to thetarget load current I_(TRGT) and to regulate the regulator voltageV_(REG) across the series combination of the regulation FET Q232 and thefeedback resistor R242 to the target regulator voltage V_(REG-TRGT). Thecontrol circuit 140 may calculate the peak magnitude I_(PK) of the loadcurrent I_(LOAD) from the magnitude of the load current feedback signalV_(ILOAD) using equations 1 and 2 shown above. If the peak magnitudeI_(PK) of the load current I_(LOAD) is not equal to the target loadcurrent I_(TRGT) at step 326, or if the regulator voltage V_(REG) (asdetermined from the LED voltage feedback signal V_(LED-NEG)) is notequal to the target regulator voltage V_(REG-TRGT) at step 328, thecontrol circuit 140 executes the current load control procedure 500 onceagain. The control circuit 140 continues to execute the current loadcontrol procedure 500 until the load current I_(LOAD) is equal to thetarget load current I_(TRGT) at step 326 and the regulator voltageV_(REG) is equal to the target regulator voltage V_(REG-TRGT) at step328.

When the peak magnitude I_(PK) of the load current I_(LOAD) is equal tothe target load current I_(TRGT) at step 326 and the regulator voltageV_(REG) is equal to the target regulator voltage V_(REG-TRGT) at step328, the control circuit 140 determines if the dimming method is the PWMdimming technique at step 330. If not, the startup procedure 300 simplyexits. However, if the dimming method is the PWM dimming technique atstep 330, the control circuit 140 sets the duty cycle DC_(DIM) of thedimming control signal V_(DIM) equal to a target dimming duty cycleDC_(TRGT) at step 332 to control the intensity of the LED light source102 to the target intensity L_(TRGT) and the startup procedure 300exits.

If the target load current I_(TRGT) or the dimming method is not known(i.e., is not stored in the memory 170) at steps 314, 316, the controlcircuit 140 changes to the CCR dimming mode at step 334 and sets theduty cycle DC_(DIM) of the dimming control signal V_(DIM) equal to thehigh-end dimming duty cycle DC_(HE) and the target load current I_(TRG)equal to the minimum load current I_(LOAD-MIN) (e.g., approximately twomilliamps) at step 336. The control circuit 140 then regulates the loadcurrent I_(LOAD) to be equal to the minimum load current I_(LOAD-MIN)using the current load control procedure 500, before the startupprocedure 300 exits. If the LED driver 100 is operating in the voltageload control mode at step 312 and the target load voltage V_(TRGT) isnot known (i.e., not stored in the memory 170) at step 338, the controlcircuit 140 changes to the current load control mode at step 340. Thecontrol circuit 140 then changes to the CCR dimming mode at step 334 andsets the duty cycle DC_(DIM) of the dimming control signal V_(DIM) tothe high-end dimming duty cycle DC_(HE) and the target load currentI_(TRGT) to the minimum load current I_(LOAD-MIN) at step 336, beforethe control circuit 140 regulates the load current I_(LOAD) to theminimum load current I_(LOAD-MIN) using the current load controlprocedure 500 and the startup procedure 300 exits. Because at least oneof the target load current I_(TRGT) and the dimming method is not known,the control circuit 140 controls the flyback converter 120 and the LEDdrive circuit 130 to provide the minimum amount of current to the LEDlight source 102 such that the LED light source is not damaged by beingexposed to excessive voltage or current.

Referring to FIG. 4B, if the LED driver 100 is operating in the voltageload control mode at step 312 and the target load voltage V_(TRGT) isknown (i.e., stored in the memory 170) at step 338, the control circuit140 determines a current limit I_(LIMIT) to which the load currentI_(LOAD) will be limited during the voltage load control mode.Specifically, if a maximum power dissipation P_(MAX) divided by thetarget load voltage V_(TRGT) is less than a maximum load current I_(MAX)at step 342, the control circuit 140 sets the current limit I_(LIMIT) tobe equal to the maximum power dissipation P_(MAX) divided by the targetload voltage V_(TRGT) at step 344. Otherwise, the control circuit 140sets the current limit I_(LIMIT) to be equal to the maximum load currentI_(MAX) at step 346. At step 348, the control circuit 140 sets the dutycycle DC_(IPK) of the peak current control signal V_(IPK) to a maximumpeak current duty cycle DC_(MAX) (i.e., 100%). At step 350, the controlcircuit 140 sets the duty cycle DC_(DIM) of the dimming control signalV_(DIM) equal to the low-end dimming duty cycle DC_(LE), such that thedimming FET Q250 is rendered conductive 99% of the time, and theregulation FET Q232 is rendered conductive approximately 1% of the time.

Next, the control circuit 140 regulates the load voltage V_(LOAD) acrossthe LED light source 102 to the target load voltage V_(TRGT) using avoltage load control procedure 600 (which will be described in greaterdetail below with reference to FIG. 7). If the load voltage V_(LOAD) isnot equal to the target load voltage V_(TRGT) at step 352, the controlcircuit 140 executes the voltage load control procedure 600 once again.When the load voltage V_(LOAD) is equal to the target load voltageV_(TRGT) at step 352, the control circuit 140 sets the duty cycleDC_(DIM) of the dimming control signal V_(DIM) equal to the targetdimming duty cycle DC_(TRGT) at step 354 to control the intensity of theLED light source 102 to the target intensity L_(TRGT) and the startupprocedure 300 exits.

FIG. 5 is a simplified flowchart of a target intensity procedure 400executed by the control circuit 140 of the LED driver 100 (when both thetarget load current I_(TRGT) or the dimming method are known). Thecontrol circuit 140 executes the target intensity procedure 400 when thetarget intensity L_(TRGT) changes at step 410, for example, in responseto a change in the DC magnitude of the target intensity control signalV_(TRGT) generated by the phase-control input circuit 160. If the LEDdriver 100 is operating in the current load control mode (as stored inthe memory 170) at step 412, the control circuit 140 determines at step414 if the LED driver is using the PWM dimming technique (as stored inthe memory 170). If so, the control circuit 140 adjusts the duty cycleDC_(DIM) of the dimming control signal V_(DIM) at step 416 in responseto the new target intensity L_(TRGT), so as to control the intensity ofthe LED light source 102 to the new target intensity L_(TRGT). If theLED driver 100 is operating in the current load control mode at step 412and with the CCR dimming technique at step 414, the control circuit 140adjusts the target load current I_(TRGT) of the load current I_(LOAD) inresponse to the new target intensity L_(TRGT) at step 418 before thetarget intensity procedure 400 exits. Specifically, the control circuit140 adjusts the duty cycle DC_(IPK) of the peak current control signalV_(IPK) at step 418, so as to control the magnitude of the load currentI_(LOAD) towards the target load current I_(TRGT). If the LED driver 100is operating in the voltage load control mode at step 412, the controlcircuit 140 adjusts the duty cycle DC_(DIM) of the dimming controlsignal V_(DIM) in response to the new target intensity L_(TRGT) at step416 and the target intensity procedure 400 exits.

FIG. 6 is a simplified flowchart of the current load control modeprocedure 500, which is executed periodically by the control circuit 140when the LED driver 100 is operating in the current load control mode.The current load control mode procedure 500 allows the control circuit140 to regulate the peak magnitude I_(PK) of the load current I_(LOAD)flowing through the feedback resistor R242 to the target load currentI_(TRGT) and to control the magnitude of the regulator voltage V_(REG)across the series combination of the regulation FET Q232 and thefeedback resistor R242 by controlling the magnitude of the bus voltageV_(BUS). For example, the control circuit 140 may determine the peakmagnitude of the load current I_(LOAD) from the average magnitudeI_(AVE) of the load current I_(LOAD) and the duty cycle DC_(DIM) of thedimming control signal V_(DIM), i.e., I_(PK)=I_(AVE)/(1−DC_(DIM)), asshown in Equation 2 above. If the peak magnitude I_(PK) of the loadcurrent I_(LOAD) is less than the target load current I_(TRGT) at step510, the control circuit 140 increases the duty cycle DC_(IPK) of thepeak current control signal V_(IPK) by a predetermined percentageΔDC_(IPK) at step 512. Accordingly, the magnitude of the gate voltageV_(IPK-G) at the gate of the regulation FET Q232 will increase, thuscausing the peak magnitude I_(PK) of the load current I_(LOAD) toincrease. If the load current I_(LOAD) is not less than the target loadcurrent I_(TRGT) at step 510, but is greater than the target loadcurrent I_(TRGT) at step 514, the control circuit 140 decreases the dutycycle DC_(IPK) of the peak current control signal V_(IPK) by thepredetermined percentage ΔDC_(IPK) at step 516 to decrease the peakmagnitude I_(PK) of the load current I_(LOAD).

Next, the control circuit 140 adjusts the magnitude of the bus voltageV_(BUS) in order to minimize the regulator voltage V_(REG) to minimizethe power dissipation in the FET Q232, while ensuring that the regulatorvoltage does not drop too low and the load voltage V_(LOAD) does nothave any voltage ripple. Specifically, if the regulator voltage V_(REG)(as determined from the LED voltage feedback signal V_(LED-NEG)) isgreater than the target regulator voltage V_(REG-TRGT) at step 518, thecontrol circuit 140 increases the duty cycle DC_(BUS) of the bus voltagecontrol signal V_(BUS-CNTL) by the predetermined percentage ΔDC_(BUS) atstep 520 to decrease the magnitude of the bus voltage V_(BUS) and thusdecrease the magnitude of the regulator voltage V_(REG). If theregulator voltage V_(REG) is not greater than the target regulatorvoltage V_(REG-TRGT) at step 518, but is less than the target regulatorvoltage V_(REG-TRGT) at step 522, the control circuit 140 decreases theduty cycle DC_(BUS) of the bus voltage control signal V_(BUS-CNTL) bythe predetermined percentage ΔDC_(BUS) at step 524 to increase themagnitude of the bus voltage V_(BUS) to ensure that the regulatorvoltage V_(REG) does not drop too low. If the load current I_(LOAD) isequal to the target load current I_(TRGT) at steps 510, 514, and theregulator voltage V_(REG) is equal to the target regulator voltageV_(REG-TRGT) at steps 518, 522, the current load control mode procedure500 simply exits without adjusting the duty cycle DC_(IPK) of the peakcurrent control signal V_(IPK) or the duty cycle DC_(BUS) of the busvoltage control signal V_(BUS-CNTL).

FIG. 7 is a simplified flowchart of a voltage load control modeprocedure 600, which is executed periodically by the control circuit 140when the LED driver 100 is operating in the voltage load control mode.The voltage load control mode procedure 600 allows the control circuit140 to regulate the load voltage V_(LOAD) to the target load voltageV_(TRGT) by controlling the magnitude of the bus voltage V_(BUS). If themagnitude of the load current I_(LOAD) is less than the current limitI_(LIMIT) at step 610, the control circuit 140 subtracts the magnitudeof the regulator voltage V_(REG) (as represented by the LED voltagefeedback signal V_(LED-NEG)) from the magnitude of the bus voltageV_(BUS) (as represented by the bus voltage feedback signal V_(BUS-FB))at step 612 to calculate the magnitude of the load voltage V_(LOAD). Ifthe load voltage V_(LOAD) is less than the target load voltage V_(TRGT)at step 614, the control circuit 140 decreases the duty cycle DC_(BUS)of the bus voltage control signal V_(BUS-CNTL) using aproportional-integral-derivative (PID) control technique at step 616 tothus increase the magnitude of the bus voltage V_(BUS), before thevoltage load control mode procedure 600 exits. If the load voltageV_(LOAD) is not less than the target load voltage V_(TRGT) at step 614,but is greater than the target load voltage V_(TRGT) at step 618, thecontrol circuit 140 increases the duty cycle DC_(BUS) of the bus voltagecontrol signal V_(BUS-CNTL) using the PID control technique at step 620to thus decrease the magnitude of the bus voltage V_(BUS), before thevoltage load control mode procedure 600 exits. If the load voltageV_(LOAD) is not less than the target load voltage V_(TRGT) at step 614and is not greater than the target load voltage V_(TRGT) at step 618(i.e., the load voltage V_(LOAD) is equal to the target load voltageV_(TRGT)), the voltage load control mode procedure 600 exits withoutadjusting the duty cycle DC_(BUS) of the bus voltage control signalV_(BUS-CNTL).

If the magnitude of the load current I_(LOAD) is greater than or equalto the current limit I_(LIMIT) at step 610, the control circuit 140begins to operate in an overcurrent protection mode at step 622 in orderto limit the load current I_(LOAD) to be less than the current limitI_(LIMIT). For example, the control circuit 140 may decrease the dutycycle DC_(IPK) of the peak current control signal V_(IPK) until the loadcurrent I_(LOAD) becomes less than the current limit I_(LIMIT) at step624. During the overcurrent protection mode, the load voltage V_(LOAD)may drop lower than the target load voltage V_(TRGT). The controlcircuit continues to operate in the overcurrent protection mode at step622 while the magnitude of the load current I_(LOAD) remains greaterthan or equal to the current limit I_(LIMIT) at step 624. When themagnitude of the load current I_(LOAD) decreases below the current limitI_(LIMIT) at step 624, the control circuit 140 executes the startupprocedure 300 (as shown in FIGS. 4A and 4B) and the voltage load controlmode procedure 600 exits.

FIG. 8 is a simplified schematic diagram of an LED drive circuit 730 ofan LED driver 700 according to a second embodiment of the presentinvention. The LED drive circuit 730 is controlled by a control circuit740 in response to the peak current control signal V_(IPK) in a similarmanner as the control circuit 130 controls the LED drive circuit 130 ofthe first embodiment. In the current load control mode, the controlcircuit 740 is operable to control the peak magnitude I_(PK) of the loadcurrent I_(LOAD) to range from approximately the minimum load currentI_(LOAD-MIN) to the maximum load current I_(LOAD-MAX) to dim the LEDlight source 102 across the dimming range. According to the secondembodiment of the present invention, the maximum load currentI_(LOAD-MAX) is at least one hundred times greater than the minimum loadcurrent I_(LOAD-MIN). For example, the minimum load current I_(LOAD-MIN)may be approximately two milliamps, and the maximum load currentI_(LOAD-MAX) may be approximately two amps, such that the maximum loadcurrent I_(LOAD-MAX) is one thousand times greater than the minimum loadcurrent I_(LOAD-MIN)

The LED drive circuit 730 comprises a regulation FET Q732 coupled inseries with the LED light source 102 for controlling the magnitude ofthe load current I_(LOAD) conducted through the LED light source 102.The LED drive circuit 730 comprises a filter circuit 734 that receivesthe peak current control signal V_(IPK) from the control circuit 740 andgenerates the filtered peak current control signal V_(IPK-F).Specifically, the filter circuit 734 comprises a two-stage RC filterhaving two resistors R738A, R739A (e.g., both having resistances ofapproximately 10 kΩ) and two capacitors C738B, C739B (e.g., both havingcapacitances of approximately 1 μF). As shown in FIG. 8, the filtercircuit 734 is referenced to the source of the regulation FET Q732. Thefiltered peak current control signal V_(IPK-F) is coupled to the gate ofthe regulation FET Q732 via an amplifier circuit 736 and a resistor R735(e.g., having a resistance of approximately 150Ω). The amplifier circuit736 may have, for example, a gain x of approximately one, such that theamplifier circuit simply operates as a buffer.

The LED drive circuit 730 also comprises a dimming FET Q750, which iscontrolled in response to the dimming control signal V_(DIM) from thecontrol circuit 140 to dim the LED light source 102 using the PWMdimming technique (in a similar manner as the dimming FET Q250 of thefirst embodiment is controlled). An NPN bipolar junction transistor Q752is coupled between the filter circuit 732 and the amplifier circuit 734for selectively coupling the filtered peak current control signalV_(IPK-F) to the amplifier circuit. The dimming FET Q750 is coupled tothe base of the transistor Q752 via a resistor R754 (e.g., having aresistance of approximately 100 kΩ). A resistor R756 is coupled betweenthe emitter and the base of the transistor Q752 and has, for example, aresistance of approximately 100 kΩ. When the dimming FET Q750 iscontrolled to be conductive, the transistor Q752 is also renderedconductive, thus coupling the filtered peak current control signalV_(IPK-F) to the amplifier circuit 734, such that the regulation FETQ732 is controlled to be conductive. When the dimming FET Q750 iscontrolled to be non-conductive, the transistor Q752 is also renderednon-conductive and the filtered peak current control signal V_(IPK-F) isnot provided to the amplifier circuit 734, such that the regulation FETQ732 is rendered non-conductive.

The LED drive circuit 730 comprises a current mirror circuit R760, whichis coupled across the LED light source 102 and generates a load voltagefeedback signal V_(LOAD-FB) representative of the magnitude of the loadvoltage V_(LOAD). The control circuit 740 receives the load voltagefeedback signal V_(LOAD-FB), such that the control circuit does not needto calculate the magnitude of the load voltage by subtracting themagnitude of the regulator voltage V_(REG) from the magnitude of the busvoltage V_(BUS) (as in the first embodiment). The load voltage feedbacksignal V_(LOAD-FB) is also provided to an inverting input of acomparator 762 for providing over-voltage protection for the LED drivecircuit 730. When the magnitude of the load voltage feedback signalV_(LOAD-FB) exceeds the magnitude of a first reference voltage V_(REF1),the comparator 762 is operable to pull the gate of the regulation FETQ732 down towards circuit common, thus rendering the regulation FET Q732non-conductive and controlling the load voltage V_(LOAD) toapproximately zero volts. The magnitude of the first reference voltageV_(REF1) corresponds to a magnitude of the load voltage V_(LOAD) thatrepresents an over-voltage condition for the LED light source 102. Forexample, the magnitude of the first reference voltage V_(REF1) may bechosen such that the regulation FET Q732 is rendered non-conductive whenthe magnitude of the load voltage V_(LOAD) exceeds approximately 40volts for a Class 2 LED light source.

The LED drive circuit 730 comprises an adjustable gain feedback circuit770 that allows the control circuit 740 to properly measure the peakmagnitude I_(PK) of the load current I_(LOAD) from the minimum loadcurrent I_(LOAD-MIN) to the maximum load current I_(LOAD-MAX), which maybe approximately one thousand times greater than the minimum loadcurrent I_(LOAD-MIN). The adjustable gain feedback circuit 770 comprisesa filter circuit 746 and an amplifier circuit 748 for generating theload current feedback signal V_(ILOAD) (in a similar manner as thefilter circuit 246 and the amplifier circuit 248 of the feedback circuit242 of the first embodiment). The amplifier circuit 748 may comprise anon-inverting operational amplifier circuit having a gain y (e.g.,approximately 20). The adjustable gain feedback circuit 770 iscontrolled to adjust the magnitude of the load current feedback signalV_(ILOAD) in response to a gain control signal V_(GAIN) generated by thecontrol circuit 740 when operating in the current load control mode. Theadjustable gain feedback circuit 770 comprises two feedback resistorsR772, R774, which are coupled in series with the regulation FET Q732(i.e., to replace the feedback resistor R244 of the feedback circuit 242of the first embodiment). For example, the resistors R772, R774 may haveresistances of approximately 0.0375Ω and 1.96Ω, respectively. A FET Q775is coupled across the second feedback resistor R774 and is controlled tobe conductive and non-conductive to control the gain (i.e., themagnitude) of the load current feedback signal V_(ILOAD). The gaincontrol signal V_(GAIN) is coupled to the gate of the FET Q775 via adrive circuit comprising a FET Q776 and two resistors R778, R779 (e.g.,having resistances of approximately 5 kΩ and 1 kΩ, respectively).

According to the second embodiment of the present invention, the gaincontrol signal V_(GAIN) is controlled so as to adjust the equivalentresistance RFB of the adjustable gain feedback circuit 770 (to thusincrease the gain of the adjustable gain feedback circuit) when themagnitude of the load current I_(LOAD) is less than or equal to athreshold current I_(TH) (e.g., approximately 100 mA). The magnitude ofthe load current I_(LOAD) crosses the threshold current I_(TH) in themiddle of the dimming range of the LED driver 700. When the magnitude ofthe load current I_(LOAD) is less than or equal to the threshold currentI_(TH), the gain control signal V_(GAIN) is controlled to be high (i.e.,at approximately the third supply voltage V_(CC3)), such that the FETQ776 is rendered conductive and the gate of the FET Q775 is pulled downtowards circuit common. Accordingly, the FET Q775 is renderednon-conductive, and both the first and second feedback resistors R772,R774 (i.e., approximately 2Ω total resistance) is coupled in series withthe regulation FET Q732. When the magnitude of the load current I_(LOAD)is greater than the threshold current I_(TH), the gain control signalV_(GAIN) is controlled to be low (i.e., at approximately circuit common)rendering the FET Q776 non-conductive, such that the gate of the FETQ775 is pulled up towards the second supply voltage V_(CC2), and the FETQ775 is rendered conductive. Thus, only the first feedback resistor R772(i.e., approximately 0.0375Ω) is coupled in series with the regulationFET Q732. For example, the control circuit 740 may control the gaincontrol signal V_(GAIN) using some hysteresis, such that the FET Q775 isnot quickly and unstably rendered conductive and non-conductive.

When the FET Q775 of the adjustable gain feedback circuit 770 isrendered conductive and non-conductive, there is a step change in theresistance coupled in series with the regulation FET Q732 (and thus astep change in the magnitude of the voltage at the source of theregulation FET). As a result, there may also be a sharp change in theload current I_(LOAD), which could cause a slight and temporary increaseor decrease (e.g., a “blip”) in the intensity of the LED light source102. Because the threshold current I_(TH) is in the middle of thedimming range of the LED driver 100, it is very desirable to have nofluctuations of the intensity of the LED light source 102 as theintensity of the LED light source is being dimmed up or dimmed down.Since the filter circuit 732 is referenced to the source of theregulation FET Q732, changes in the magnitude of the voltage at thesource do not greatly affect the magnitude of the peak current controlsignal V_(IPK) and thus the gate-source voltage of the regulation FETQ732. Accordingly, the large fluctuations of the load current I_(LOAD)(and thus the intensity of the LED light source 102) are minimized whenthe 1-ET Q775 is rendered conductive and non-conductive at the thresholdcurrent I_(TH).

In addition, the control circuit 740 “pre-loads” the peak currentcontrol signal V_(IPK) whenever the magnitude of the load currentI_(LOAD) transitions above or below the threshold current I_(TH) toavoid large fluctuations of the load current I_(LOAD) and thus theintensity of the LED light source 102. Specifically, when the magnitudeof the load current I_(LOAD) transitions across the threshold currentI_(TH), the control circuit 740 enters a transition mode in which theclosed loop control of the regulation FET Q732 (i.e., the current loadcontrol procedure 500) is paused. After entering the transition mode,the control circuit 740 adjusts the peak current control signal V_(IPK)by a predetermined correction factor ΔV_(IPK), and then waits for afirst delay time T_(DELAY1) approximately one to two milliseconds)before controlling the gain control signal V_(GAIN) to render the FETQ775 either conductive or non-conductive. After controlling the FETQ775, the control circuit 740 waits for a second delay time T_(DELAY2)after which the control circuit exits the transition mode and resumesthe close loop control of the regulation FET Q732. For example, thesecond delay time T_(DELAY2) may be approximately ten milliseconds whenthe magnitude of the load current I_(LOAD) has transitioned above thethreshold current I_(TH) and approximately four milliseconds when themagnitude of the load current I_(LOAD) has transitioned below thethreshold current I_(TH).

Referring back to FIG. 8, the LED drive circuit 730 further comprises anover-current protection circuit having an amplifier circuit 764 (e.g.,having a gain z of approximately two) and a comparator 766. When themagnitude of load current I_(LOAD) increases such that the magnitude ofthe voltage at the non-inverting input of the comparator 766 exceeds themagnitude of a second reference voltage V_(REF2), the comparator 766 isoperable to pull the gate of the regulation FET Q732 down towardscircuit common, thus rendering the regulation FET Q732 non-conductiveand controlling the load current I_(LOAD) to approximately zero amps.The magnitude of the second reference voltage V_(REF2) corresponds to amagnitude of the load current I_(LOAD) that represents an over-currentcondition through the LED light source 102. For example, the magnitudeof the second reference voltage V_(REF2) may be chosen such that theregulation FET Q732 is rendered non-conductive when the magnitude of theload current I_(LOAD) exceeds approximately four amps.

FIG. 9 is a simplified flowchart of a transition mode procedure 800executed periodically by the control circuit 740 when the LED driver 700is operating in the current load control mode. During the transitionmode procedure 800, the control circuit 740 begins operating in atransition mode if the magnitude of the load current I_(LOAD) has justtransitioned across the threshold current I_(TH). If the control circuit740 is not in the transition mode at step 810 when the transition modeprocedure 800 begins, the control circuit first executes the currentload control procedure 500 (as shown in FIG. 6). For example, thecontrol circuit 740 may calculate the peak magnitude I_(PK) of the loadcurrent I_(LOAD) using Equations 1 and 2 shown above, where theequivalent resistance R_(FB) of the adjustable-gain feedback circuit 770is dependent upon the state of the FET Q775. For example, the equivalentresistance R_(FB) may be equal to approximately the resistance of theresistor R772 when the FET Q775 is conductive, and may be equal toapproximately the resistance of the series combination of the first andsecond feedback resistors R772, R774 when the FET Q775 isnon-conductive.

After executing the current load control mode procedure 500, the controlcircuit 740 then checks to determine if the magnitude of the loadcurrent I_(LOAD) just transitioned across the threshold current I_(TH).Specifically, if the magnitude of the load current I_(LOAD) has risenabove the threshold current I_(TH) at step 812, the control circuit 740adds the correction factor ΔV_(IPK) to the peak current control signalV_(IPK) at step 814 and enters the transition mode at step 816 (i.e.,execution of the current load control procedure 500 is paused). Thecontrol circuit 740 then initializes a first delay timer to the firstdelay time T_(DELAY1) and starts the first delay timer decreasing invalue with respect to time at step 818, before the transition modeprocedure 800 exits. If the magnitude of the load current I_(LOAD) hasjust dropped below the threshold current I_(TH) at step 820, the controlcircuit 740 subtracts the correction factor ΔV_(IPK) from the peakcurrent control signal V_(IPK) at step 822, enters the transition modeat step 816, and starts the first delay timer with the first delay timeT_(DELAY1) at step 818, before the transition mode procedure 800 exits.

When the control circuit 740 is in the transition mode at step 810, thecontrol circuit 740 does not executed the current load control procedure500, and rather operates to control the FET Q775 to adjust the gain ofthe adjustable-gain feedback circuit 770. Specifically, when the firstdelay timer expires at step 824 and the magnitude of the load currentI_(LOAD) has risen above the threshold current I_(TH) at step 826, thecontrol circuit 740 drives the gain control signal V_(GAIN) low at step828 to render the FET Q775 conductive, such that only the first feedbackresistor R772 is coupled in series with the regulation FET Q732. Thecontrol circuit 740 then updates the equivalent resistance R_(FB) of theadjustable gain feedback circuit 770 to be equal to the resistance ofonly the resistor R772 at step 830. At step 832, the control circuit 740initializes a second delay timer to the second delay time T_(DELAY2) andstarts the second delay timer decreasing in value with respect to time,before the transition mode procedure 800 exits.

When the first delay timer expires at step 824 and the magnitude of theload current I_(LOAD) has dropped below the threshold current I_(TH) atstep 826, the control circuit 740 drives the gain control signalV_(GAIN) high at step 834 to render the FET Q775 non-conductive, suchthat both the first and second feedback resistors R772, R774 are coupledin series with the regulation FET Q732. The control circuit 740 thenadjusts resistance R_(FB) of the adjustable gain feedback circuit 770 tobe equal to the resistance of the series combination of the resistorsR772, R774 at step 830, and starts the second delay timer with thesecond delay time T_(DELAY2) at step 832, before the transition modeprocedure 800 exits. When the second delay timer expires at step 836,the control circuit 740 exits the transition mode at step 838, such thatwhen the transition mode procedure 800 is executed again, the currentload control procedure 500 will be executed.

FIG. 10 is a simplified block diagram of an LED driver 900 according toa third embodiment of the present invention. The LED driver 900 of thethird embodiment includes many similar functional blocks as the LEDdriver 100 of the first embodiment as shown in FIG. 2. However, the LEDdriver 900 of the third embodiment does not include the power supply150. Rather, the LED driver 900 comprises a buck-boost flyback converter920, which generates the variable DC bus voltage V_(BUS) across the buscapacitor C_(BUS), as well as generating the various DC supply voltagesV_(CC1), V_(CC2), V_(CC3) for powering the circuitry of the LED driver.

In addition, the LED drive circuit 930 includes a multiple-outputfeedback circuit 970 (FIG. 12) that provides first and second loadcurrent feedback signals V_(ILOAD1), V_(ILOAD2) to a control circuit940. The first load current feedback signal V_(ILOAD1) is characterizedby a first gain γ₁ applied to the average magnitude I_(AVE) of the loadcurrent I_(LOAD), while the second load current feedback signalV_(ILOAD2) is characterized by a second gain γ₂. The second gain γ₂(e.g., approximately 101) is greater than the first gain γ₁ (e.g.,approximately one), such that the first and second load current feedbacksignals V_(ILOAD1), V_(ILOAD2) provide two differently scaledrepresentations of the average magnitude I_(AVE) of the load currentI_(LOAD). The control circuit 940 uses both of the first and second loadcurrent feedback signals V_(ILOAD1), V_(ILOAD2) to determine the peakmagnitude I_(PK) of the load current I_(LOAD), which may range from theminimum load current I_(LOAD-MIN) to the maximum load currentI_(LOAD-MAX) (as will be described in greater detail below).Accordingly, the maximum load current I_(LOAD-MAX) may be at least onehundred times greater than the minimum load current I_(LOAD-MIN), forexample, approximately one thousand times greater than the minimum loadcurrent I_(LOAD-MIN), as in the second embodiment.

FIG. 11 is a simplified circuit diagram of the flyback converter 920 ofthe LED driver 900 of the third embodiment of the present invention. Theflyback converter 920 comprises a flyback transformer 910 having aprimary winding coupled in series with a FET Q912 and a feedbackresistor R926. The secondary winding of the flyback transformer 910 iscoupled to the bus capacitor C_(BUS) via a diode D914. The secondarywinding of the flyback transformer 910 comprises a center tap thatgenerates a center tap voltage V_(TAP) having a magnitude proportionalto the magnitude of the bus voltage V_(BUS). The bus voltage feedbacksignal V_(BUS-FB) is generated by a voltage divider comprising tworesistors R916, R918 coupled across the bus capacitor C_(BUS) and isprovided to the control circuit 140. The center tap voltage V_(TAP) isused to generate the second supply voltage V_(CC2) and the third supplyvoltage V_(CC3) as will be described in greater detail below.

The flyback converter 920 comprises a flyback controller 922, whichoperates in a similar manner as the flyback controller 222 of theflyback converter 120 of the first embodiment to generate the busvoltage V_(BUS) across the bus capacitor C_(BUS). The flyback controller922 controls the FET Q912 in response to the bus voltage control signalV_(BUS-CNTL) received from the control circuit 140 (via a filter circuit924 and an optocoupler circuit 926) and a control signal received fromthe feedback resistor R928 and representative of the current through theFET Q912.

The flyback converter 920 comprises a flyback controller power supply932 for generating the first DC supply voltage V_(CC1) (e.g.,approximately 14 volts for powering the flyback controller 922) across acapacitor C929 (e.g., having a capacitance of approximately 220 μF). Theflyback controller power supply 932 is coupled to a supply winding 910Aof the flyback transformer 910, such that the flyback controller powersupply is only able to generate the first DC supply voltage V_(CC1)while the flyback converter 920 is actively generating the DC busvoltage V_(BUS) (i.e., after the flyback controller 922 has started up).The flyback controller power supply 932 comprises a pass-transistorsupply that includes an NPN bipolar junction transistor Q934, a resistorR935 (e.g., having a resistance of approximately 10 kΩ), a zener diodeZ936 (e.g., having a breakover voltage of approximately 14 volts), and adiode D938. The emitter of the transistor Q934 is coupled to thecapacitor C929 through the diode D938 and the zener diode Z936 iscoupled to the base of the transistor Q934. Accordingly, the capacitorC929 is able to charge through the transistor Q934 to a voltage equal toapproximately the break-over voltage of the zener diode Z936 minus thebase-emitter drop of the transistor and the diode drop of the diodeD938.

Since the flyback controller power supply 932 is only able to generatethe first DC supply voltage V_(CC1) while the flyback converter 920 isactively generating the DC bus voltage V_(BUS), the flyback converterfurther comprises a startup power supply 950 for allowing the capacitorC929 to charge before the flyback controller 922 has started up. Thestartup power supply 950 comprises a cat-ear power supply including aFET Q952 for allowing the capacitor C929 to charge from the rectifiedvoltage V_(RECT) through a diode D954 and a resistor R956 (e.g., havinga resistance of approximately 1Ω). The gate of the FET Q952 is coupledto the rectified voltage V_(RECT) through two resistors R958, R960(e.g., having resistances of approximately 250 kΩ and 200 kΩ,respectively), such that shortly after the beginning of a half-cycle ofthe AC power source 104, the FET 952 is rendered conductive allowing thecapacitor C929 to charge. An NPN bipolar junction transistor Q962 iscoupled to the gate of the FET Q952 for providing over-currentprotection in the startup power supply 950. Specifically, if the currentthrough the FET Q952 increases such that the voltage across the resistorR956 exceeds the rated base-emitter voltage of the transistor Q962, thetransistor Q962 becomes conductive, thus rendering the FET 952non-conductive.

The gate of the FET Q952 is coupled to circuit common via an NPN bipolarjunction transistor Q964. The base of the transistor Q964 is coupled tothe rectified voltage V_(RECT) via the resistor R958, a zener diode Z965(e.g., having a breakover voltage of approximately 5.6 volts), andanother resistor R966 (e.g., having a resistance of approximately 1 MΩ).A resistor R968 is coupled between the base and the emitter of thetransistor Q964 and has, for example, a resistance of approximately 392kΩ. When the magnitude of the rectified voltage V_(RECT) increases to amagnitude such that the voltage across the resistor R968 exceeds thebreakover voltage of the zener diode Z965 and the base-emitter voltageof the transistor Q964, the transistor Q964 is rendered conductive, thuspulling the gate of the FET 952 down towards circuit common.Accordingly, the FET 952 is rendered non-conductive preventing thecapacitor C929 from charging from the rectified voltage V_(RECT). As aresult, the startup power supply 950 only allows the capacitor C929 tocharge around the zero-crossings of the AC power source 104, and thusprovide more efficient operation during startup of the flybackcontroller 922 than, for example, simply having a single resistorcoupled between the rectified voltage V_(RECT) and the capacitor C929.After the capacitor C929 has appropriately charged (i.e., the magnitudeof the first DC supply voltage V_(CC1) has exceeded the rated operatingvoltage of the flyback controller 922), the flyback controller powersupply 932 is able to generate the first DC supply voltage V_(CC1) andthe startup power supply 950 ceases operating. However, the startuppower supply 950 may once again begin operating during normal operationif the voltage across the supply winding 910A drops below approximatelythe first DC supply voltage V_(CC1).

The flyback converter 920 further comprises first and second powersupplies 980, 990 that have outputs that are coupled together. The firstand second power supplies 980, 990 operate separately (e.g., in acomplementary fashion) to generate the second DC supply voltage V_(CC2)across a capacitor C972 (e.g., having a capacitance of approximately 0.1μF) during different modes of operation of the LED driver 900. The firstpower supply 980 is coupled to the center tap of the flyback transformer910 through a diode D974, and draws current from a capacitor C976, whichis coupled to the input of the first power supply and has a capacitanceof, for example, approximately 220 μF. The second power supply 990 iscoupled to the bus voltage V_(BUS) and thus draws current from the buscapacitor C_(BUS). A linear regulator 999 receives the second DC supplyvoltage V_(CC2) and generates the third DC supply voltage V_(CC3) acrossan output capacitor C978 (e.g., having a capacitance of approximately2.2 μF).

The magnitude of the bus voltage V_(BUS) is controlled by the controlcircuit 940 to optimize the efficiency and reduce the total powerdissipation of the LED driver 100 during the current load control modeprocedure 500, and to regulate the load voltage V_(LOAD) to the targetload voltage V_(TRGT) in a similar manner as the control circuit 140 ofthe first embodiment (i.e., during the voltage load control modeprocedure 600). When the magnitude of the center tap voltage V_(TAP) isabove a cutover voltage V_(CUT) (e.g., approximately 10 volts), thefirst power supply 980 operates charge the capacitor C972 (rather thanthe second power supply 990). When the magnitude of the center tapvoltage V_(TAP) is below the cutover voltage V_(CUT), the first powersupply 980 stops charging the capacitor C972, and the second powersupply 990 operates to charge the capacitor C972. Accordingly, theflyback converter 920 provides a wide output range and only a singlehigh-frequency switching transistor (i.e., FET Q912) in addition togenerating the three DC supply voltages V_(CC1), V_(CC2), V_(CC3).

Both of the power supplies 980, 990 comprise pass-transistor supplies.The first power supply 980 comprises a NPN bipolar junction transistorQ982 coupled between the diode D974 and the capacitor C972 forconducting current to the capacitor C972. The first power supply 980further comprises a resistor R984, which is coupled between thecollector and the emitter of the transistor Q982 and has, for example, aresistance of approximately 10 kΩ. A diode D985 and a zener diode Z986(e.g., having a breakover voltage of approximately 10 volts) are coupledin series between the base of the transistor Q982 and circuit common,such that the capacitor C972 is able to charge to a voltage equal toapproximately the breakover voltage of the zener diode. A diode D988 iscoupled from the emitter to the collector of the transistor Q982, suchthat when the transistor Q982 is non-conductive, the voltage across thecapacitor C972 is maintained at approximately a diode drop below thesecond DC supply voltage V_(CC2).

The second power supply 990 comprises an NPN bipolar junction transistorQ992 coupled between the bus voltage V_(BUS) and the capacitor C972 anda resistor R994, which is coupled between the collector and the base ofthe transistor Q992 and has a resistance of, for example, approximately10 kΩ. The second power supply 990 further comprises a zener diode Z996coupled between the base of the transistor Q992 and circuit common, suchthat the capacitor C972 is operable to charge through the transistorQ992 to a voltage equal to approximately the breakover voltage of thezener diode minus the base-emitter voltage of the transistor Q992. Whenthe magnitude of the center tap voltage V_(TAP) drops below the cutovervoltage V_(CUT) and the diode D988 of the first power supply 980 becomesforward biased, the second power supply 990 begins to generate thesecond DC supply voltage V_(CC2). Since the zener diode Z986 of thefirst power supply 980 and the zener diode Z996 of the second powersupply 990 have the same breakover voltage (i.e., approximately 10volts), the second power supply could alternatively not comprise thezener diode Z996 and the first and second power supplies could “share”the zener diode Z986. Specifically, the base of the transistor Q992 ofthe second power supply 990 would be coupled to the junction of thediode D985 and the zener diode Z986 of the first power supply 980.

FIG. 12 is a simplified schematic diagram of the LED drive circuit 930of the LED driver 900 according to the third embodiment of the presentinvention. As previously mentioned, the LED driver circuit 930 comprisesthe multiple-output feedback circuit 970 that generates the two loadcurrent feedback signals V_(ILOAD1), V_(ILOAD2). The control circuit 940is able to control the NET Q775 to either couple only the resistor R772or the series combination of the resistors R772, R774 in series with theregulation FET Q732. The first load current feedback signal V_(ILOAD1)is produced by the filter circuit 746, and is thus simply a filteredversion of the voltage generated across the feedback circuit 970 (i.e.,the voltage across either the resistor R772 or the series combination ofthe resistors R772, R774 depending upon the state of the FET Q775). Inother words, the first gain γ₁ of the first load current feedback signalV_(ILOAD1) is approximately one. The second load current feedback signalV_(ILOAD2) is an amplified version of the voltage generated across thefeedback circuit 970, i.e., as generated by an amplifier circuit 948,such that the second gain γ₂ of the second load current feedback signalV_(ILOAD2) is approximately 101. In other words, the magnitude of thesecond load current feedback signal V_(ILOAD2) is approximately equal tothe magnitude of the first load current feedback signal V_(ILOAD1)multiplied by the second gain γ₂.

The control circuit 940 is operable to appropriately control theregulation FET Q732 in response to both of the load current feedbacksignals V_(ILOAD1), V_(ILOAD2). Specifically, the control circuit 940uses the first load current feedback signal V_(ILOAD1) to determine thepeak magnitude I_(PK) of the load current I_(LOAD) when the magnitude ofthe second load current feedback signal V_(ILOAD2) is above a maximumvoltage threshold V_(TH-MAX). The control circuit 940 uses the secondload current feedback signal V_(ILOAD2) to determine the peak magnitudeI_(PK) of the load current I_(LOAD) when the magnitude of the secondload current feedback signal V_(ILOAD2) is below a minimum voltagethreshold V_(TH-MIN). For example, the maximum and minimum voltagethresholds V_(TH-MAX), V_(TH-MIN) may be approximately 3 volts and 2.95volts respectively. In other words, the control circuit 940 only usesthe second load current feedback signal V_(ILOAD2) to determine the peakmagnitude I_(PK) of the load current I_(LOAD) when the magnitude of thesecond load current feedback signal V_(ILOAD2) is less than 2.95 volts,which is less than a rated maximum voltage (e.g., approximately 3.3volts) of the microprocessor of the control circuit 940. When themagnitude of the second load current feedback signal V_(ILOAD2) exceeds3 volts (and also may exceed the rated maximum voltage of themicroprocessor), the control circuit 940 then uses the first loadcurrent feedback signal V_(ILOAD1) (which has a magnitude less than therated maximum voltage of the microprocessor) to determine the peakmagnitude I_(PK) of the load current I_(LOAD).

When the magnitude of the second load current feedback signal V_(ILOAD2)is between the maximum voltage threshold V_(TH-MAX) and the minimumvoltage threshold V_(TH-MIN), the control circuit 940 “slushes” (i.e.,combines) the first and second load current feedback signals V_(ILOAD1),V_(ILOAD2) together to determine a value to use for the magnitude of theload current I_(LOAD). Specifically, the control circuit 940 calculatesthe magnitude of the load current I_(LOAD) using a weighted sum of thefirst and second current feedback signals V_(ILOAD1), V_(ILOAD2), wherethe values of weight factors m and n are each a function of themagnitude of the second load current feedback signal V_(ILOAD2).Alternatively, the values of the weight factors could each be a functionof the magnitude of the first load current feedback signal V_(ILOAD1).In addition, the values of the weight factors could each alternativelybe recalled from a look-up table, or could be calculated as a functionof the elapsed time since the magnitude of either of the first andsecond load current feedback signals V_(ILOAD1), V_(ILOAD2) droppedbelow the maximum voltage threshold V_(TH-MAX) or rose above the minimumvoltage threshold V_(TH-MIN).

According to the third embodiment of the present invention, the controlcircuit 940 does not control the gain control signal V_(GAIN) to controlthe FET Q775 during normal operation of the LED driver 900. In otherwords, the control circuit 940 does not render the FET Q775 conductiveand non-conductive depending upon the magnitude of the load currentI_(LOAD) at some point in the middle of the dimming range. The memory170 of the LED driver 900 of the third embodiment is programmed at thetime of manufacture to either render the FET Q775 conductive ornon-conductive at all times during operation. Even though the gaincontrol signal V_(GAIN) is not adjusted during normal operation of theLED driver 900 (and is only adjusted at the time of manufacture), theFET Q775 still allows a single piece of electrical hardware to be usedto control LED light sources having a plurality of different ratedvoltages and/or rated currents.

FIG. 13 is a simplified flowchart of a load current feedback procedure1000, which is executed periodically by the control circuit 940 when theLED driver 900 is operating in the current load control mode. If themagnitude of the second load current feedback signal V_(ILOAD2) isgreater than the maximum voltage threshold V_(TH-MAX) at step 1010, thecontrol circuit 940 calculates the peak magnitude I_(PK) of the loadcurrent I_(LOAD) as a function of the magnitude of the first loadcurrent feedback signal V_(ILOAD1) at step 1012, e.g.,I _(PK) =f(V _(ILOAD1))=V _(ILOAD1)/[(1−DC _(DIM))·γ₁ ·R_(FB))].  (Equation 3)

The control circuit 940 then executes the current load control procedure500 using the peak magnitude I_(PK) of the load current I_(LOAD) asdetermined at step 1012, before the load current feedback procedure 1000exits. If the magnitude of the second load current feedback signalV_(ILOAD2) is less than the minimum voltage threshold V_(TH-MIN) at step1014, the control circuit 940 calculates the peak magnitude I_(PK) ofthe load current I_(LOAD) as a function of the magnitude of the secondload current feedback signal V_(ILOAD2) at step 1016, e.g.,I _(PK) =f(V _(ILOAD2))=V _(ILOAD2)/[(1−DC _(DIM))·γ₂ ·R_(FB))],  (Equation 4)and then executes the current load control procedure 500, before theload current feedback procedure 1000 exits.

If the magnitude of the second load current feedback signal V_(ILOAD2)is not greater than the maximum voltage threshold V_(TH-MAX) at step1010 and is not less than the minimum voltage threshold V_(TH-MIN) atstep 1014, the control circuit 940 calculates the first weight factor mas a function of the magnitude of the second load current feedbacksignal V_(ILOAD2) at step 1018, e.g.,

$\begin{matrix}{m = {\frac{V_{{ILOAD}\; 2} - V_{{TH}\text{-}{MIN}}}{V_{{TH}\text{-}{MAX}} - V_{{TH}\text{-}{MIN}}}.}} & ( {{Equation}\mspace{14mu} 5} )\end{matrix}$

The control circuit 940 then calculates the second weight factor n fromthe first weight factor m (i.e., also as a function of the magnitude ofthe second load current feedback signal V_(ILOAD2)) at step 1020, e.g.,n=1−m.  (Equation 6)

The control circuit 940 then uses the weighting factors m, n tocalculate the peak magnitude I_(PK) of the load current I_(LOAD) as afunction of the weighted sum of the first and second load currentfeedback signals V_(ILOAD1), V_(ILOAD2) at step 1022, e.g.,I _(PK) =[m·V _(ILOAD1) +n·V _(ILOAD2) /y]/[(1−DC _(DIM))·R_(FB)].  (Equation 7)

The control circuit 940 then executes the current load control procedure500 using the peak magnitude I_(PK) of the load current I_(LOAD) asdetermined at step 1022, before the load current feedback procedure 1000exits.

According to an alternative embodiment of the present invention, thecontrol circuit 940 of the LED driver 900 could control the gain controlsignal V_(GAIN) to control the FET Q775 during normal operation (as inthe second embodiment) in addition to receiving both of the first andsecond load current feedback signals V_(ILOAD1), V_(ILOAD2) (as in thethird embodiment) in order to achieve an even greater dimming range.

FIG. 14 is a simplified schematic diagram of an LED drive circuit 1130of a LED driver 1100 according to a fourth embodiment of the presentinvention. The LED driver 1100 of the fourth embodiment comprises acontrol circuit 1140 that is operable to control the intensity of theLED light source 102 using a combined PWM-CCR dimming technique whenoperating in the current load control mode. FIG. 15A is a plot of theduty cycle DC_(ILOAD) of the load current I_(LOAD) with respect to thetarget intensity L_(TRGT) of the LED light source 102 according to thefourth embodiment of the present invention. FIG. 15B is a plot of thepeak magnitude I_(PK) of the load current I_(LOAD) conducted through theLED light source 102 with respect to the target intensity L_(TRGT) ofthe LED light source 102 according to the fourth embodiment of thepresent invention.

When the target intensity L_(TRGT) of the LED light source 102 is abovea threshold intensity L_(TH), the LED driver 1100 regulates the peakmagnitude I_(PK) of the load current I_(LOAD) to a maximum peakmagnitude I_(PK-MAX), and operates using the PWM dimming technique toonly adjust the duty cycle DC_(ILOAD) of the load current I_(LOAD). Forexample, the threshold intensity L_(TH) may be dependent upon thesmallest value of the duty cycle DC_(DIM) of the dimming control signalV_(DIM) that the control circuit 1140 can generate. The control circuit940 is operable to adjust the intensity of the LED light source 102below the threshold intensity L_(TH) by decreasing the peak magnitudeI_(PK) of the load current I_(LOAD). Specifically, the LED driver 1100maintains the duty cycle DC_(ILOAD) of the load current I_(LOAD)constant at a minimum duty cycle DC_(ILOAD-MIN) (e.g., approximately1-5%), and reduces the peak magnitude I_(PK) of the load currentI_(LOAD) (towards a minimum peak magnitude I_(PK-MIN)) as the targetintensity L_(TRGT) of the LED light source 102 decreases below thethreshold intensity L_(TH).

The LED drive circuit 1130 comprises an adjustable gain feedback circuit1170 that does not include a filter circuit (i.e., the filter circuit746 of the LED drive circuit 730 of the second embodiment). Therefore,the adjustable gain feedback circuit 1170 generates a load currentfeedback signal V_(ILOAD)′ that is provided to a control circuit 1140and is representative of the instantaneous magnitude I_(INST) of theload current I_(LOAD) (rather than the average magnitude T_(AVE)). Abovethe threshold intensity L_(TH), the control circuit 1140 is operable tocontrol the dimming control signal V_(DIM) to adjust the duty cycleDC_(ILOAD) of the pulse-width modulated load current I_(LOAD) and thusthe intensity of the LED light source 102. Below the threshold intensityL_(TH), the control circuit 1140 is operable to control the peak currentcontrol signal V_(IPK) to adjust the peak magnitude I_(PK) of thepulse-width modulated load current I_(LOAD) and thus the intensity ofthe LED light source 102.

The control circuit 1140 is also operable to control the FET Q775 toadjust the gain of the adjustable gain feedback circuit 1170 when thepeak magnitude I_(PK) of the load current I_(LOAD) crosses a peakcurrent threshold I_(PK-TH) (for example, using some hysteresis). Afterthe peak magnitude I_(PK) of the load current I_(LOAD) transitionsacross the peak current threshold I_(PK-TH), the control circuit 1140 isoperable to render the FET Q775 of the adjustable gain feedback circuit1170 conductive and non-conductive during one of the “valleys” of thepulse-width modulated load current I_(LOAD), i.e., when the dimmingcontrol signal V_(DIM) is low and the regulation FET Q732 isnon-conductive, such that the instantaneous magnitude I_(INST) of theload current I_(LOAD) is approximately zero amps. By controlling the FETQ775 during the valleys of the pulse-width modulated load currentI_(LOAD), the control circuit 1140 is operable to avoid largefluctuations of the load current I_(LOAD) and thus the intensity of theLED light source 102 while dimming the LED light source.

FIG. 16 is a simplified flowchart of a target intensity procedure 1200executed by the control circuit 1140 of the LED driver 1100 when thetarget intensity L_(TRGT) changes at step 1210 according to the fourthembodiment of the present invention. If the new target intensityL_(TRGT) is greater than or equal to the threshold intensity L_(TH) atstep 1212, the control circuit 1140 controls the duty cycle DC_(IPK) ofthe peak current control signal V_(IPK) to control the peak magnitudeI_(PK) of the load current I_(LOAD) to the maximum peak magnitudeI_(PK-MAX) at step 1214. At step 1216, the control circuit 1140 adjuststhe duty cycle DC_(DIM) of the dimming control signal V_(DIM) inresponse to the new target intensity L_(TRGT), so as to control theintensity of the LED light source 102 to the new target intensityL_(TRGT), and the target intensity procedure 1200 exits. If the newtarget intensity L_(TRGT) is less than the threshold intensity L_(TH) atstep 1212, the control circuit 1140 controls the duty cycle DC_(DIM) ofthe dimming control signal V_(DIM) at step 1218, so as to maintain theduty cycle DC_(ILOAD) of the load current I_(LOAD) at the minimum dutycycle DC_(ILOAD-MIN). At step 1220, the control circuit 1140 adjusts thetarget load current I_(TRGT) of the load current I_(LOAD) in response tothe new target intensity L_(TRGT), and the target intensity procedure1200 exits.

FIG. 17 is a simplified flowchart of a transition mode procedure 1300executed periodically by the control circuit 1140 according to thefourth embodiment of the present invention. The control circuit 1140first executes the current load control procedure 500 (as shown in FIG.6). According to the fourth embodiment, the control circuit 1140 isoperable to calculate the peak magnitude I_(PK) of the load currentI_(LOAD) from the load current feedback signal V_(ILOAD)′ when thedimming control signal V_(DIM) is high (and the instantaneous magnitudeI_(INST) of the load current I_(LOAD) is greater than approximately zeroamps), i.e.,I _(PK) =I _(INST) =V _(ILOAD)′/(β·R _(FB)).  (Equation 8)

During the transition mode procedure 1300, the control circuit 1140begins operating in a transition mode if the peak magnitude I_(PK) ofthe load current I_(LOAD) has just transitioned across the peak currentthreshold I_(PK-TH). Specifically, if the control circuit 1140 is not inthe transition mode at step 1310, but the peak magnitude I_(PK) of theload current I_(LOAD) has just transitioned across the peak currentthreshold I_(PK-TH) at step 1312, the control circuit 1140 beginsoperating in a transition mode at step 1314.

Next the control circuit 1140 waits until the dimming control signalV_(DIM) is low (i.e., at approximately circuit common), such that theinstantaneous magnitude I_(INST) of the load current I_(LOAD) isapproximately zero amps, before controlling the FET Q775 to adjust thegain of the adjustable-gain feedback circuit 1170. Specifically, whenthe control circuit 1140 is operating in the transition mode at step1310 or at step 1314, but the dimming control signal V_(DIM) is not lowat step 1316, the transition mode procedure 1300 simply exits. However,when the dimming control signal V_(DIM) is low at step 1316 and the peakmagnitude I_(PK) of the load current I_(LOAD) has risen above thethreshold current I_(TH) at step 1318, the control circuit 1140 drivesthe gain control signal V_(GAIN) low at step 1320 to render the FET Q775conductive, such that only the first feedback resistor R772 is coupledin series with the regulation FET Q732. The control circuit 1140 thenupdates the equivalent resistance R_(FB) of the adjustable gain feedbackcircuit 1170 to be equal to the resistance of only the resistor R772 atstep 1322 and exits the transition mode at step 1324, before thetransition mode procedure 1300 exits. When the magnitude of the loadcurrent I_(LOAD) has dropped below the threshold current I_(TH) at step1318, the control circuit 1140 drives the gain control signal V_(GAIN)high at step 1326 to render the FET Q775 non-conductive, such that boththe first and second feedback resistors R772, R774 are coupled in serieswith the regulation FET Q732.

FIG. 18 shows an exemplary LED driver configuration system 1400 forconfiguring the LED drivers 100, 700, 900, 1100 according to anembodiment of the present invention. The configuration system 1400 canbe used in multiple locations including a lamp/LED driver manufacturingfacility (i.e., a factory); an original equipment manufacturing (OEM)site where a lighting fixture may be preassembled with the LED driver(e.g., LED driver 100), lamp load (e.g., LED light source 102), and/oran a lighting control (e.g., dimmer switch 106); or in the field, i.e.,at the lighting system installation location to optimize the lightingsystem driver to the installed lighting system. The system utilizessoftware (e.g., a configuration program) that can be downloaded from aserver connected to the Internet 1410. Alternatively, the software couldbe provided on a storage medium such as a disc or CD. The configurationprogram, which allows the user to program the operating characteristicsof the lamp driver, such as the LED driver 100, is loaded into apersonal computer (PC) 1420, and will be described in further detailbelow. According to an embodiment of the present invention, the userinteracts with the configuration program using a graphical userinterface (GUI) software to select the operating mode and voltage and/orcurrent at which the configurable LED driver 100 will operate the LEDlight source 102.

The configuration program that is loaded into the computer 1420 allowsthe user to select the operational mode (current load control mode orvoltage load control mode) as well as the dimming technique (e.g.,constant current reduction, constant current PWM, or constant voltagePWM) and incrementally change the magnitude of the current or voltage atwhich the LED driver 100 will operate the LED light source 102. Thesoftware operating on the computer 1420 will provide instructions to aprogramming device 1450 via, for example, a universal serial bus (USB)port 1422 and a USB jack 1424. The programming device 1450 is providedwith power from the AC power source 102 via a standard line cord 1460,or could alternatively be provided with power from a DC supply, abattery supply, or from the USB jack 1424. The programming device 1450converts the instructions received from the computer 1420 on the USBport 1422 to data that is provided via a terminal block 1426 to the LEDdriver 100 (i.e., to the communication circuit 180) via a communicationbus 1430.

In order to provide feedback to the computer 1420 during theconfiguration process, an optional sensor 1470 can be provided tomeasure different characteristics of the LED driver 100 and/or the LEDlight source 102. For example, the sensor 1470 may comprise aphotosensor that measures the light output of the LED light source 102and provides a signal back to the computer 1420, and the measured lightoutput may be displayed on the computer such that the user can determineif a desired light level has been reached. Alternatively, the sensor1470 may further comprise a power meter along with the photosensor whichcould be operable to provide “lumen per watt” feedback to the user. Thesensor 1470 could alternatively comprise a temperature sensor thatmeasures the temperature of the LED driver 100 and/or the LED lightsource 102, and sends that information to the computer 1420 such thatthe user can be advised of the operating temperature(s). The sensor 1470could further be operable to measure the color temperature and/or thecolor rendering index of the LED light source 102 and provide thatinformation to the user on the computer 1420 such that the user canconfigure the LED driver to achieve a desired color characteristic. Theprocess for measuring different characteristics of the LED driver 100with the sensor 1470 could be automated (e.g., provided as a “wizard”)to assist the user in optimizing a certain characteristic of the LEDdriver. Alternatively, feedback can be dispensed with, in which case theuser can manually adjust the operating characteristics of the LED driver100 such that the desired performance is achieved visually.

FIG. 19 is a simplified block diagram of the programming device 1450.The programming data from the computer 1420 that is used to program theLED driver 100 according to the desired operation mode and dimmingtechnique and to the target voltage or current, is transmitted via theUSB jack 1424 to a USB-to-RS232 interface 1490. The USB-to-RS232interface 1490 translates the USB serial data into RS232 serial format,and is powered by the USB connection from the computer 1420. The outputof the USB-to-RS232 interface 1490 is provided to a further interface1495 that translates the RS232 data into the LED driver 100 protocolutilized on the communication bus 1430 to which the LED driver 100 isconnected, for example, the Lutron ECOSYSTEM communication protocolwhich allows a plurality of drivers (or fluorescent lamp ballasts andother devices such as sensors) to communicate with each other on thecommunication bus 1430. The programming device 1450 comprises a buspower supply 1497 for powering the communication bus 1430. The bus powersupply 1497 is powered from the AC power source 104 via the line cord1460. A low voltage supply 1499 provides power for the interface 1495from the AC power source 104 via the line cord 1460. Alternatively, thelow voltage supply 1499 could receive power via the USB jack 1424. Theprogramming device 1450 can be used in the factory, at a fixture OEMsite, or in the field to program the LED driver 100. Although theembodiment described utilizes the USB, RS232, and driver protocols,these are merely illustrative. Any other communication protocols,standards, or specifications can be used, as desired, such as, but notlimited to, wireless communication.

FIG. 20 shows an example GUI screen display 1480 on the computer 1420,and FIG. 21 is a general flowchart 1500 of the operation of the lampdriver configuration system 1400. To use the lamp driver configurationsystem 1400, the user first downloads the configuration program from theInternet by connecting the computer 1420 to the manufacturer's websiteat step 1510. Alternatively, the configuration program could beotherwise obtained (e.g., on a storage medium, such as a compact disc)and then loaded into the computer 1420. Next, the communication bus 1430is connected to the LED driver 100 and to the terminal block 1426 of theprogramming device 1450 at step 1512 to allow the LED driver to beprogrammed with the settings provided by the computer 1420. After theLED driver 100 is connected to the terminal block 1426 of theprogramming device 1450, power is applied to the LED driver by turningon the AC power source 104 at step 1514 (e.g., by closing a circuitbreaker or operating a switch or dimmer switch connected to the AC powersource). Next, the user uses the GUI software of the configurationprogram running on the computer 1420 to set the parameters (i.e.,control mode and desired current and/or voltage) for the LED driver 100at step 1516. The parameters are then sent to the programming device1450 and thus to the LED driver 100 to program the LED driver with theseparameters at step 1518 (i.e., the parameter are saved in memory 170 ofthe LED driver). The GUI software of the configuration program can beused to incrementally select the driver parameters until the desiredperformance is attained. If the desired performance is not achieved atstep 1520, the user may adjust the parameters of the LED driver 100 atstep 1516, and reprogram the LED driver at step 1518.

The configuration program loaded into the computer 1420 allows the userto select the operation mode and dimming technique. As previouslydiscussed, the LED driver 100 can operate in a voltage load control modeusing a PWM dimming technique, a current load control mode using a PWMdimming technique, or a current load control mode using constant currentreduction. The “output type” selection on the GUI screen display 1480allows the user to select both the operation mode and dimming techniquetogether (i.e., constant voltage PWM, constant current PWM or constantcurrent reduction). In addition, the user can dial in the desired(target) corresponding voltage or current. According to an embodiment ofthe present invention, the LED driver 100 may be provided in severalbasic models. For example, the LED driver may, in order to cover theentire output range necessary, be provided in three basic power ranges,a high range, a medium range and a low range in order to cover therequired output operational range. The base model of the LED driver 100that is used will be automatically determined during the configurationprogram. In addition to the power ranges of the LED driver 100, forwhich there may be multiple, as explained, there may also be differentphysical “form factors” for the LED driver. For example, the LED driver100 may take the form of three physically different devices, a K can, aK can with studs, and an M can device. These different form factorsprovide for different installation and mounting techniques.

As shown on the example GUI screen display 1480 of FIG. 20, the GUIsoftware allows the user to configure the LED driver 100 in one of twoways, by parameter (“by setting”) or by model number. In each case, theLED driver 100 that is connected to the programming device 1450 isidentified by the configuration software (i.e., the driver sends backits model number which includes at least a base model number).

If the user chooses to configure by setting, the user clicks on “bysetting”. The user selects the output type (constant voltage PWM,constant current PWM, or constant current reduction), sets the targetvoltage or current (depending on output type) and also selects the otherparameters (form factor, input signal, etc.). The model number isdetermined and displayed by the software in response to the enteredparameters. If the user selects a parameter not within the specificationrange of the connected driver (i.e., the base model is different thanthe connected driver base model), the base model will be highlighted onthe screen to alert the user that a different driver must be connectedor different parameters consistent with the connected driver must beconnected. So long as the selected parameters are within thespecifications of the connected driver, the software will determine themodel number which will be identified on the screen for ordering by theuser, for example, over the Internet. If the settings are inconsistentwith the connected driver, an error message will be generated and theparameters will not be saved to the LED driver 100. Assuming theconnected driver is compatible with the selected parameters, the drivercan then be programmed and/or the model number of the configured drivercan be ordered. Alternatively, even if the LED driver 100 is notconnected to the programming device 1450, the GUI software can stillallow the user to ‘build’ a model number by selecting the desiredsettings such that the appropriate LED driver may be ordered. If themodel number of the configured driver is ordered, the parameters can beprogrammed into the appropriate base model driver at the factory andshipped to the customer either for installation or for use as a sample.The programmed LED driver 100 can also be labeled with the programmedparameters. For example, a label machine may be coupled to the computer1420 and may be operable to print a label with the proper model numberand/or programmed parameters upon successfully programming an LED driver100.

If the user chooses “by model number”, a model number may be entered ormodified by the user in the “by model number” window. If the modelnumber entered is within the specification of the currently connectedLED driver 100, the currently connected driver can then be reconfiguredper the specifications of the selected model number. If the selectedmodel number is outside the specification of the currently connecteddriver, the base model number field will be highlighted, alerting theuser that the selected model number is outside the specifications of thecurrently connected driver. The user can then select a different modelnumber or restart by connecting a different base model driver.

The GUI screen display 1480 also allows the user to specify the formfactor, i.e., the particular physical form of the LED driver 100 and anyother mechanical options as well as the control input, which may be acommunication bus input (received by the communication circuit 180 ofthe LED driver) or a phase control input (received by the phase controlinput circuit 160 of the LED driver). Specifically, the phase controlinput may be either a two-wire electronic-low voltage (ELV)phase-control input or a three-wire phase-control input. In addition,the LED driver 100 may be operable to be responsive to a combination ofcontrol inputs. For example, one LED driver 100 may be configured to beoperable to receive control inputs from both the communication bus viathe communication circuit 180 and three-wire phase control dimmingsignals via the phase control input circuit 160. A safety rating may bedisplayed in response to the selections made. According to analternative embodiment, the desired safety rating may be entered by theuser. In addition, the screen will show an image of the selectedmechanical form factor of the driver at 1485.

FIG. 22 is a simplified software flowchart of a configuration process1600 executed by the computer 1420 (i.e., the GUI software) of the lampdriver configuration system 1400. The configuration process 1600 istypically started after the user has downloaded the configurationprogram, connected the LED driver 100 to the programming device 1450,and applied power to the LED driver (per steps 1510, 1512, 1514 of FIG.21). At step 1602, the configuration program waits to receive a“Connect” command in response to the user clicking the “Connect” buttonon the GUI display screen 1480 of the computer 1420. Once the user hasclicked the “Connect” button, the computer 1420 attempts to establishcommunication with the LED driver 100 via the programming device 1450 atstep 1604.

Then at step 1610, the programming device 1450 retrieves the base modelnumber from the LED driver 100. Additionally, at step 1610, theprogramming device 1450 may be operable to retrieve other parametersfrom the LED driver 100 such as output type, control input type, ormechanical form factor in the event that the LED driver had already beenprogrammed or manufactured with some parameters. At step 1612, the basemodel and/or full model number and any other parameter informationretrieved from the LED driver 100 are then displayed on the GUI display.

Next, the system waits to receive a “by setting” command at step 1614 ora “by model number” command at step 1618 in response to the user'sselection of the associated radio button on the GUI display. If the userhas selected the “by setting” radio button at step 1614, then at step1616, the user can select the desired electrical and optional parametersfor the LED driver 100 using the dropdown menus on the GUI displayscreen 1480. As the user makes various parameter selections at step1616, the model number displayed on the GUI display screen 1480 may alsoupdate in response to those parameter selections. If the user hasselected the “by model number” radio button at step 1618, then at step1620, the user can enter the complete desired model number by using thedropdown model number entry screen on the GUI display. As the userenters portions of the model number on the GUI display at step 1620, theparameter information corresponding to the entered model number are alsodisplayed on the GUI screen display 1480, and further settings may beeliminated depending on the portion of the model number entered into theGUI software.

Once the user has provided all of the necessary user input, theconfiguration program waits for a “Save to Driver” command at step 1626in response to the user clicking the “Save to Driver” button on the GUIdisplay screen 1480. If the configuration program does not receive the“Save to Driver” command at step 1626, then the process loops back tostep 1612 such that the user may make any additional changes to theselected parameters and/or model number.

If the system receives the “Save to Driver” command at step 1626, thenat step 1628, the system verifies that the selected parameters and/ormodel number are compatible with the base model number that was detectedat step 1610. If the selected parameters and/or model number are notcompatible with the detected base model, then at step 1630, the user isnotified of the incompatibility between the selected parameters and thebase model. The user may decide at step 1634 to change the LED driver100 and then to click the “Connect” button at step 1602 (i.e., toreconnect a different LED driver having the compatible base model numberto the programming device 1450). Alternatively, if the user decides notto change the connected LED driver 100 at step 1634, then the user maychange any of the incompatible selections via steps 1612-1620.

If at step 1628, the selected parameters and/or model number iscompatible with the detected base model, then at step 1632, the settingsare sent to the LED driver 100 via the programming device 1450, the LEDdriver verifies the received settings, and the user is notified that theLED driver has been programmed with the new settings. At this point, theprocess 1600 ends. However, in the event that the user evaluates therecently programmed LED driver 100 and determines that the driver is notoperating as expected, the user may easily repeat the process 1600 inorder to make any additional modifications to the LED driver 100.

FIG. 23 shows a simplified software flowchart of the configurationprocess 1700 executed by the LED driver 100. The process is executed bythe control circuit 140 of the LED driver 100 once communication hasbeen established between the programming device 1450 and the LED driver(i.e., after step 1604 of process 1600). At step 1702, the controlcircuit 140 retrieves the base model number from the memory 170. Thebase model number may be saved to the memory 170 during the initialmanufacturing process of the LED driver 100. Then, at step 1704, thecontrol circuit 140 retrieves the target voltage V_(TRGT) and/or currentI_(TRGT) from the memory 170 if known or saved. At step 1706, the outputtype (i.e., the load control mode and the dimming method) are retrievedfrom the memory 170 if known or saved. Next at step 1708, all of thedata that was retrieved from the memory 170 is sent to the programmingdevice 1450 such that it can be displayed on the GUI display screen 1480(i.e., at step 1612 of process 1600). The control circuit 140 then waitsat step 1710 to receive new parameters from the programming device 1450,and once the new parameters are received, they are stored in the memory170 at step 1712 before the process 1700 ends.

Thus, the configuration program allows the user to program the LEDdriver 100 to a desired current for a constant current driver or desiredvoltage for a constant voltage driver and change the current or voltageas desired, until the desired parameters, such as desired light output,are achieved either by visual observation or by feedback from the sensor1470 that may be connected to the user's computer. Once the desiredparameters of the LED driver 100 are achieved, the LED driver can beordered from the factory by the model number identified on the GUIdisplay (as shown on the example GUI screen display 1480 in FIG. 20)associated with the selected specification. According to an alternateembodiment, the GUI display may include an “Order Now” button whichallows the user to order the model number identified on the screen viathe Internet 1410 (i.e., on-line). In response to clicking the “OrderNow” button, the user may be presented with (on the computer 1420) anadditional order screen via the Internet 1410 where the user may provideadditional billing and shipping information such that the on-line ordercan be properly processed. In the factory, one of the basic modeldrivers can then be programmed to the selected specifications and thememory contents locked to those settings by preventing further changesto the target voltage or current stored in the microprocessor's memory.In the factory, the driver can be labeled with the selectedspecifications, i.e., operating voltage, current or power, for example,according to necessary code requirements or safety approval agencies,e.g. Underwriters Laboratory (UL).

Thus, an optimized LED driver 100 can be configured. This configurationcan be achieved to optimize the lighting system driven by the driver. Inaddition, a single LED driver 100 can be easily and quickly reconfiguredmultiple times to evaluate the overall performance of the lightingsystem. Furthermore, the computer 1420 can identify the particular modelnumber of the LED driver associated with the configured parameters. Thismodel number driver can then be either ordered by the user forinstallation or a sample can be ordered for testing at the installationlocation.

Accordingly, the development tool according to the present inventionallows the user to configure an LED driver to the optimizedconfiguration necessary for a particular application. This alsominimizes the number of LED drivers that the factory needs to stock.According to the present invention, the factory needs only stock alimited number of basic LED drivers in different power ranges, forexample, three, each in a different power range, plus a limited numberof different physical form factor variations, e.g. three, as well as alimited range of control inputs, e.g., two different control inputvariations, i.e., ELV phase control input or communication bus inputplus three wire phase control input. The factory accordingly need stockonly eighteen base models of driver that is three output ranges timesthree form factors times two control inputs for a total of eighteen basemodels. Then, using the tool according to the present invention, theappropriate base model can be programmed with the desired voltage andcurrent specifications, as selected in the field. Those voltage andcurrent specifications can then be locked in so that they cannot bealtered and the driver can be labeled with the final model according tothe programmed settings. These specifications can also be used for ULapproval.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A load control circuit for controlling the amountof power delivered to an electrical load, the load control circuitcomprising: a regulation transistor adapted to be coupled in series withthe load to control the magnitude of a load current conducted throughthe load, so as to control the amount of power delivered to the load;and a feedback circuit coupled in series with the regulation transistorand operable to generate a first load current feedback signalrepresentative of the magnitude of the load current; wherein theregulation transistor operates in the linear region in response to themagnitude of the load current determined from the first load currentfeedback signal, such that the load control circuit is able to controlthe magnitude of the load current conducted through the load from aminimum load current to a maximum load current, the maximum load currentat least approximately one thousand times larger than the minimum loadcurrent.
 2. The load control circuit of claim 1, further comprising: acontrol circuit operatively coupled to the regulation transistor forcontrolling the regulation transistor to operate in the linear region tothus adjust the magnitude of the load current through the load inresponse to the first load current feedback signal.
 3. The load controlcircuit of claim 2, wherein the feedback circuit generates a second loadcurrent feedback signal representative of the magnitude of the loadcurrent, the first and second load current feedback signalscharacterized by respective first and second gains applied to themagnitude of the load current, the first gain different than the secondgain, the control circuit operable to determine the magnitude of theload current in response to both the first and second load currentfeedback signals.
 4. The load control circuit of claim 3, wherein thecontrol circuit only uses the first load current feedback signal todetermine the magnitude of the load current when the magnitude of thesecond load current feedback signal is greater than a first thresholdvoltage, and only uses the second load current feedback signal todetermine the magnitude of the load current when the magnitude of thesecond load current feedback signal is less than a second thresholdvoltage.
 5. The load control circuit of claim 4, wherein the controlcircuit combines the first and second load current feedback signals todetermine the magnitude of the load current when the magnitude of thesecond load current feedback signal is between the first and secondthreshold voltages.
 6. The load control circuit of claim 5, wherein thecontrol circuit uses a weighted sum of the first and second load currentfeedback signals to determine the magnitude of the load current when themagnitude of the second load current feedback signal is between thefirst and second threshold voltages.
 7. The load control circuit ofclaim 6, wherein weighting factors of the weighted sum of the first andsecond load current feedback signals are functions of the magnitude ofthe second load current feedback signal.
 8. The load control circuit ofclaim 6, wherein weighting factors of the weighted sum of the first andsecond load current feedback signals are functions of the amount ofelapsed time since the magnitude of the second load current feedbacksignal transitioned across either of the first and second thresholdvoltages.
 9. The load control circuit of claim 4, wherein the secondgain is greater than the first gain.
 10. The load control circuit ofclaim 9, wherein the first gain is approximately one.
 11. The loadcontrol circuit of claim 4, wherein the control circuit combines thefirst and second load current feedback signals to determine themagnitude of the load current when the magnitude of the second loadcurrent feedback signal is between first and second threshold voltages.12. The load control circuit of claim 2, wherein the feedback circuitcomprises an adjustable-gain feedback circuit coupled in series with theregulation FET and operable to generate a load current feedback signalrepresentative of the average magnitude of the load current, theadjustable-gain feedback circuit comprising first and second resistorscoupled in series with the regulation FET, and a gain-adjustmenttransistor coupled across the second resistor, the control circuitcoupled to the adjustable-gain feedback circuit for controlling thegain-adjustment transistor to be conductive and non-conductive, suchthat the series combination of the first and second resistors is coupledin series with the regulation FET when the gain-adjustment transistor isnon-conductive, and only the first resistor is coupled in series withthe regulation FET when the gain-adjustment transistor is conductive,the control circuit rendering the gain-adjustment transistornon-conductive when the magnitude of the load current is less than athreshold current.
 13. The load control circuit of claim 12, wherein thecontrol circuit is operable to: generate a current control signal forcontrolling the regulation transistor to operate in the linear region tothus control the magnitude of the load current conducted through theload; detect that the magnitude of the load current is less than thethreshold current; subsequently pause controlling the regulationtransistor in response to the magnitude of the load current; adjust themagnitude of the current control signal by a predetermined amount;subsequently wait for a first delay time; and render the gain-adjustmenttransistor non-conductive at the end of the first delay time.
 14. Theload control circuit of claim 13, wherein the control circuit is furtheroperable to wait for a second delay time after rendering thegain-adjustment transistor non-conductive, and resume controlling theregulation transistor in response to the magnitude of the load currentat the end of the second delay time.
 15. The load control circuit ofclaim 14, further comprising: a filter circuit coupled in series betweenthe control circuit and a gate of the regulation transistor, the filtercircuit operable to receive the current control signal from the controlcircuit, the filter circuit referenced to a source of the regulationtransistor.
 16. The load control circuit of claim 12, wherein thecontrol circuit is operable to control the regulation transistor using apulse-width modulation technique to adjust the amount of power deliveredto the load, the control circuit rendering the gain-adjustmenttransistor non-conductive and conductive during a valley of thepulse-width modulated load current when the instantaneous magnitude ofthe load current is approximately zero amps.
 17. An LED driver forcontrolling an LED light source, the LED driver comprising: a powerconverter circuit operable to receive a rectified AC voltage and togenerate a DC bus voltage; a LED drive circuit operable to receive thebus voltage and to control the magnitude of a load current conductedthrough the LED light source, the LED drive circuit comprising afeedback circuit operable to generate first and second load currentfeedback signals representative of the magnitude of the load current,the first and second load current feedback signals characterized byrespective first and second gains applied to the magnitude of the loadcurrent, the first gain different than the second gain; and a controlcircuit operable to combine the first and second load current feedbacksignals, the control circuit operatively coupled to the LED drivecircuit for controlling the magnitude of the load current through theload in response to both the first and second load current feedbacksignals, such that the LED drive circuit is able to control themagnitude of the load current conducted through the load from a minimumload current to a maximum load current.
 18. The LED driver of claim 17,wherein the control circuit uses only the first load current feedbacksignal to determine the magnitude of the load current when the magnitudeof the second load current feedback signal is greater than a firstthreshold voltage, and uses only the second load current feedback signalto determine the magnitude of the load current when the magnitude of thesecond load current feedback signal is less than a second thresholdvoltage.
 19. The LED driver of claim 18, wherein the control circuitcombines the first and second load current feedback signals to determinethe magnitude of the load current when the magnitude of the second loadcurrent feedback signal is between the first and second thresholdvoltages.
 20. The LED driver of claim 19, wherein the control circuituses a weighted sum of the first and second load current feedbacksignals to determine the magnitude of the load current when themagnitude of the second load current feedback signal is between thefirst and second threshold voltages.
 21. The LED driver of claim 20,wherein the weighting factors of the weighted sum of the first andsecond load current feedback signals are functions of the magnitude ofthe second load current feedback signal.
 22. The LED driver of claim 20,wherein the weighting factors of the weighted sum of the first andsecond load current feedback signals are functions of the amount ofelapsed time since the magnitude of the second load current feedbacksignal transitioned across either of the first and second thresholdvoltages.
 23. The LED driver of claim 17, wherein the LED drive circuitfurther comprises a regulation transistor adapted to be coupled inseries with the load to control the magnitude of the load currentconducted through the load, the control circuit operatively coupled tothe regulation transistor for controlling the regulation transistor tooperate in the linear region to thus adjust the magnitude of the loadcurrent through the load in response to the first load current feedbacksignal.
 24. The LED driver of claim 23, wherein the feedback circuitcomprises an adjustable-gain feedback circuit coupled in series with theregulation FET and operable to generate a load current feedback signalrepresentative of the magnitude of the load current, the adjustable-gainfeedback circuit comprising first and second resistors coupled in serieswith the regulation FET, and a gain-adjustment transistor coupled acrossthe second resistor, the control circuit coupled to the adjustable-gainfeedback circuit for controlling the gain-adjustment transistor to beconductive and non-conductive, such that the series combination of thefirst and second resistors is coupled in series with the regulation FETwhen the gain-adjustment transistor is non-conductive, and only thefirst resistor is coupled in series with the regulation FET when thegain-adjustment transistor is conductive, the control circuit renderingthe gain-adjustment transistor non-conductive when the magnitude of theload current is less than a threshold current.
 25. The LED driver ofclaim 24, wherein the control circuit is operable to: generate a currentcontrol signal for controlling the regulation transistor to operate inthe linear region to thus control the magnitude of the load currentconducted through the LED light source; detect that the magnitude of theload current is less than the threshold current; subsequently pausecontrolling the regulation transistor in response to the magnitude ofthe load current; adjust the magnitude of the current control signal bya predetermined amount; subsequently wait for a first delay time; andrender the gain-adjustment transistor non-conductive at the end of thefirst delay time.
 26. The LED driver of claim 23, wherein the controlcircuit is operable to control the regulation transistor using apulse-width modulation technique to adjust the intensity of the LEDlight source, the control circuit rendering the gain-adjustmenttransistor non-conductive and conductive during a valley of thepulse-width modulated load current when the instantaneous magnitude ofthe load current is approximately zero amps.
 27. The LED driver of claim17, wherein the maximum load current is at least approximately onethousand times larger than the minimum load current.
 28. A load controlcircuit for controlling the amount of power delivered to an electricalload, the load control circuit comprising: a regulation transistoradapted to be coupled in series with the load to control the magnitudeof a load current conducted through the load, so as to control theamount of power delivered to the load; a feedback circuit coupled inseries with the regulation transistor and operable to generate first andsecond load current feedback signals representative of the magnitude ofthe load current, the first and second load current feedback signalscharacterized by respective first and second gains with respect to themagnitude of the load current, the first gain different than the secondgain; and a control circuit operable to combine the first and secondload current feedback signals to determine the magnitude of the loadcurrent in response to both the first and second load current feedbacksignals, the control circuit operatively coupled to the regulationtransistor for controlling the regulation transistor to operate in thelinear region to thus adjust the magnitude of the load current throughthe load in response to the magnitude of the load current determinedfrom the first and second load current feedback signals.
 29. The loadcontrol circuit of claim 28, wherein the control circuit uses only thefirst load current feedback signal to determine the magnitude of theload current when the magnitude of the second load current feedbacksignal is greater than a first threshold voltage, and uses only thesecond load current feedback signal to determine the magnitude of theload current when the magnitude of the second load current feedbacksignal is less than a second threshold voltage.
 30. The load controlcircuit of claim 29, wherein the control circuit combines the first andsecond load current feedback signals to determine the magnitude of theload current when the magnitude of the second load current feedbacksignal is between the first and second threshold voltages.
 31. The loadcontrol circuit of claim 30, wherein the control circuit uses a weightedsum of the first and second load current feedback signals to determinethe magnitude of the load current when the magnitude of the second loadcurrent feedback signal is between the first and second thresholdvoltages.
 32. The load control circuit of claim 31, wherein theweighting factors of the weighted sum of the first and second loadcurrent feedback signals are functions of the magnitude of the secondload current feedback signal.
 33. The load control circuit of claim 31,wherein the weighting factors of the weighted sum of the first andsecond load current feedback signals are functions of the amount ofelapsed time since the magnitude of the second load current feedbacksignal transitioned across either of the first and second thresholdvoltages.
 34. The load control circuit of claim 30, wherein the secondgain is greater than the first gain.
 35. The load control circuit ofclaim 34, wherein the first gain is approximately one.
 36. The loadcontrol circuit of claim 28, wherein the control circuit combines thefirst and second load current feedback signals to determine themagnitude of the load current when the magnitude of the second loadcurrent feedback signal is between first and second threshold voltages.37. The load control circuit of claim 28, where the load control circuitis able to control the amount of power delivered to the load from aminimum load current to a maximum load current, the maximum load currentat least approximately one thousand times larger than the minimum loadcurrent.
 38. A load control circuit for controlling the amount of powerdelivered to an electrical load, the load control circuit comprising: aregulation transistor adapted to be coupled in series with the load tocontrol the magnitude of a load current conducted through the load, soas to control the amount of power delivered to the load; anadjustable-gain feedback circuit coupled in series with the regulationFET and operable to generate a load current feedback signalrepresentative of the magnitude of the load current, the adjustable-gainfeedback circuit comprising first and second resistors coupled in serieswith the regulation FET, and a gain-adjustment transistor coupled acrossthe second resistor; a control circuit operatively coupled to theregulation transistor for controlling the regulation transistor to thusadjust the magnitude of the load current through the load, the controlcircuit further coupled to the adjustable-gain feedback circuit forrendering the gain-adjustment transistor conductive and non-conductive,such that the series combination of the first and second resistors iscoupled in series with the regulation FET when the gain-adjustmenttransistor is non-conductive, and only the first resistor is coupled inseries with the regulation FET when the gain-adjustment transistor isconductive; and wherein the control circuit is operable to: generate acurrent control signal for controlling the regulation transistor tooperate in the linear region to thus control the magnitude of the loadcurrent conducted through the load; detect that the magnitude of theload current is less than the threshold current; subsequently pausecontrolling the regulation transistor in response to the magnitude ofthe load current; adjust the magnitude of the current control signal bya predetermined amount; subsequently wait for a first delay time; andrender the gain-adjustment transistor non-conductive at the end of thefirst delay time.
 39. The load control circuit of claim 38, wherein thecontrol circuit is further operable to wait for a second delay timeafter rendering the gain-adjustment transistor non-conductive, andresume controlling the regulation transistor in response to themagnitude of the load current at the end of the second delay time. 40.The load control circuit of claim 39, further comprising: a filtercircuit coupled in series between the control circuit and a gate of theregulation transistor, the filter circuit operable to receive thecurrent control signal from the control circuit, the filter circuitreferenced to a source of the regulation transistor.
 41. The loadcontrol circuit of claim 38, wherein the control circuit is operable tocontrol the regulation transistor using a pulse-width modulationtechnique to adjust the amount of power delivered to the load, thecontrol circuit rendering the gain-adjustment transistor non-conductiveand conductive during a valley of the pulse-width modulated load currentwhen the instantaneous magnitude of the load current is approximatelyzero amps.
 42. The load control circuit of claim 38, wherein theregulation transistor comprises a FET.
 43. The load control circuit ofclaim 38, where the load control circuit is able to control the amountof power delivered to the load from a minimum load current to a maximumload current, the maximum load current at least approximately onethousand times larger than the minimum load current.
 44. A method ofcontrolling the amount of power delivered to an electrical load, themethod comprising: controlling the magnitude of a load current conductedthrough the load, so as to control the amount of power delivered to theload; generating first and second load current feedback signalsrepresentative of the magnitude of the load current, the first andsecond load current feedback signals characterized by respective firstand second gains applied to the magnitude of the load current, the firstgain different than the second gain; combining the first and second loadcurrent feedback signals; calculating the magnitude of the load currentin response to combining both the first and second load current feedbacksignals; and adjusting the magnitude of the load current in response tothe calculated magnitude of the load current determined from the firstand second load current feedback signals.
 45. The method of claim 44,further comprising: using only the first load current feedback signal todetermine the magnitude of the load current when the magnitude of thesecond load current feedback signal is greater than a first thresholdvoltage; and using only the second load current feedback signal todetermine the magnitude of the load current when the magnitude of thesecond load current feedback signal is less than a second thresholdvoltage.
 46. The method of claim 45, wherein calculating the magnitudeof the load current in response to combining both the first and secondload current feedback signals further comprises combining the first andsecond load current feedback signals when the magnitude of the secondload current feedback signal is between the first and second thresholdvoltages.
 47. The method of claim 46, wherein combining the first andsecond load current feedback signals further comprises using a weightedsum of the first and second load current feedback signals to determinethe magnitude of the load current when the magnitude of the second loadcurrent feedback signal is between the first and second thresholdvoltages.
 48. The method of claim 47, wherein the weighting factors ofthe weighted sum of the first and second load current feedback signalsare functions of the magnitude of the second load current feedbacksignal.
 49. The method of claim 47, wherein the weighting factors of theweighted sum of the first and second load current feedback signals arefunctions of the amount of elapsed time since the magnitude of thesecond load current feedback signal transitioned across either of thefirst and second threshold voltages.
 50. The method of claim 45, whereinthe second gain is greater than the first gain.
 51. The method of claim50, wherein the first gain is approximately one.
 52. A method ofcontrolling the amount of power delivered to an electrical load, themethod comprising: controlling the magnitude of a load current conductedthrough the load by controlling a regulation transistor coupled so as toconduct the load current to operate in the linear region, so as tocontrol the amount of power delivered to the load; conducting the loadcurrent through first and second series-connected resistors; generatinga load current feedback signal across the series-connected resistors,the load current feedback signal representative of the magnitude of theload current; the load current feedback signal comprising a first loadcurrent feedback signal developed across a first resistor and a secondload current feedback signal developed across both resistors in series;calculating the magnitude of the load current in response to both loadcurrent feedback signals; adjusting the magnitude of the load current inresponse to the magnitude of the load current determined from the firstand second load current feedback signals; controlling a gain-adjustmenttransistor coupled across the second resistor to be conductive, suchthat the first load current feedback signal is generated from only thefirst resistor; generating a current control signal for controlling theregulation transistor to operate in the linear region to thus controlthe magnitude of the load current conducted through the load; detectingthat the magnitude of the load current is less than the thresholdcurrent; subsequently pausing controlling the regulation transistor inresponse to the magnitude of the load current; adjusting the magnitudeof the current control signal by a predetermined amount; subsequentlywaiting for a first delay time; and rendering the gain-adjustmenttransistor non-conductive at the end of the first delay time, such thatthe load current feedback signal is generated across the seriescombination of the first and second resistors.
 53. The method of claim52, further comprising: waiting for a second delay time after renderingthe gain-adjustment transistor non-conductive; and resuming controllingthe regulation transistor in response to the magnitude of the loadcurrent at the end of the second delay time.
 54. The method of claim 52,further comprising: controlling the regulation transistor using apulse-width modulation technique to adjust the amount of power deliveredto the load; and rendering the gain-adjustment transistor non-conductiveand conductive during a valley of the pulse-width modulated load currentwhen the instantaneous magnitude of the load current is approximatelyzero amps.
 55. A load control circuit for controlling the amount ofpower delivered to an electrical load, the load control circuitcomprising: a regulation transistor adapted to be coupled in series withthe load to control the magnitude of a load current conducted throughthe load, so as to control the amount of power delivered to the load; acontrol circuit operatively coupled to the regulation transistor, thecontrol circuit generating a current control signal for controlling theregulation transistor to operate in the linear region to thus adjust themagnitude of the load current through the load and an adjustable-gainfeedback circuit coupled in series with the regulation transistor andoperable to generate a load current feedback signal representative ofthe magnitude of the load current, the adjustable-gain feedback circuitcharacterized by a first gain and characterized by a second gain;wherein the control circuit is operable to: control the adjustable-gainfeedback circuit to be characterized by the first gain; detect that themagnitude of the load current is less than a threshold current;subsequently pause controlling the regulation transistor in response tothe magnitude of the load current; adjust the magnitude of the currentcontrol signal by a predetermined amount; subsequently wait for a firstdelay time; and control the adjustable-gain feedback circuit to becharacterized by the second gain.